#line 1 "main.cpp"
/**
* @title Template
*/
#include <iostream>
#include <algorithm>
#include <utility>
#include <numeric>
#include <vector>
#include <array>
#include <cassert>
#include <queue>
#include <set>
#line 2 "/Users/kodamankod/Desktop/Programming/Library/other/chmin_chmax.cpp"
template <class T, class U>
constexpr bool chmin(T &lhs, const U &rhs) {
if (lhs > rhs) {
lhs = rhs;
return true;
}
return false;
}
template <class T, class U>
constexpr bool chmax(T &lhs, const U &rhs) {
if (lhs < rhs) {
lhs = rhs;
return true;
}
return false;
}
/**
* @title Chmin/Chmax
*/
#line 2 "/Users/kodamankod/Desktop/Programming/Library/other/range.cpp"
#line 4 "/Users/kodamankod/Desktop/Programming/Library/other/range.cpp"
class range {
public:
class iterator {
private:
int64_t M_position;
public:
constexpr iterator(int64_t position) noexcept: M_position(position) { }
constexpr void operator ++ () noexcept { ++M_position; }
constexpr bool operator != (iterator other) const noexcept { return M_position != other.M_position; }
constexpr int64_t operator * () const noexcept { return M_position; }
};
class reverse_iterator {
private:
int64_t M_position;
public:
constexpr reverse_iterator(int64_t position) noexcept: M_position(position) { }
constexpr void operator ++ () noexcept { --M_position; }
constexpr bool operator != (reverse_iterator other) const noexcept { return M_position != other.M_position; }
constexpr int64_t operator * () const noexcept { return M_position; }
};
private:
const iterator M_first, M_last;
public:
constexpr range(int64_t first, int64_t last) noexcept: M_first(first), M_last(std::max(first, last)) { }
constexpr iterator begin() const noexcept { return M_first; }
constexpr iterator end() const noexcept { return M_last; }
constexpr reverse_iterator rbegin() const noexcept { return reverse_iterator(*M_last - 1); }
constexpr reverse_iterator rend() const noexcept { return reverse_iterator(*M_first - 1); }
};
/**
* @title Range
*/
#line 2 "/Users/kodamankod/Desktop/Programming/Library/other/rev.cpp"
#include <type_traits>
#include <iterator>
#line 6 "/Users/kodamankod/Desktop/Programming/Library/other/rev.cpp"
template <class T>
class rev_impl {
public:
using iterator = typename std::decay<T>::type::reverse_iterator;
private:
const iterator M_begin;
const iterator M_end;
public:
constexpr rev_impl(T &&cont) noexcept: M_begin(std::rbegin(cont)), M_end(std::rend(cont)) { }
constexpr iterator begin() const noexcept { return M_begin; }
constexpr iterator end() const noexcept { return M_end; }
};
template <class T>
constexpr decltype(auto) rev(T &&cont) {
return rev_impl<T>(std::forward<T>(cont));
}
/**
* @title Reverser
*/
#line 2 "/Users/kodamankod/Desktop/Programming/Library/other/random_number.cpp"
#include <cstdint>
#include <random>
#include <chrono>
#line 7 "/Users/kodamankod/Desktop/Programming/Library/other/random_number.cpp"
#include <type_traits>
uint64_t engine() {
static const auto rotate = [](const uint64_t x, const size_t k) {
return (x << k) | (x >> (64 - k));
};
static auto array = [] {
uint64_t seed = static_cast<uint64_t>(std::chrono::system_clock::now().time_since_epoch().count());
std::array<uint64_t, 4> res{};
for (size_t index = 0; index < 4; index++) {
uint64_t value = (seed += 0x9e3779b97f4a7c15);
value = (value ^ (value >> 30)) * 0xbf58476d1ce4e5b9;
value = (value ^ (value >> 27)) * 0x94d049bb133111eb;
res[index] = value ^ (value >> 31);
}
return res;
}();
const uint64_t result = rotate(array[1] * 5, 7) * 9;
const uint64_t old_value = array[1] << 17;
array[2] ^= array[0];
array[3] ^= array[1];
array[1] ^= array[2];
array[0] ^= array[3];
array[2] ^= old_value;
array[3] = rotate(array[3], 45);
return result;
}
template <class Integer>
typename std::enable_if<std::is_integral<Integer>::value, Integer>::type random_number(Integer lower, Integer upper) {
static std::default_random_engine gen(engine());
return std::uniform_int_distribution<Integer>(lower, upper)(gen);
}
template <class Real>
typename std::enable_if<!std::is_integral<Real>::value, Real>::type random_number(Real lower, Real upper) {
static std::default_random_engine gen(engine());
return std::uniform_real_distribution<Real>(lower, upper)(gen);
}
/**
* @title Random Number
*/
#line 20 "main.cpp"
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
constexpr i32 inf32 = (i32(1) << 30) - 1;
constexpr i64 inf64 = (i64(1) << 62) - 1;
int main() {
i32 N, M;
std::cin >> N >> M;
std::vector<i32> B(M), P(M);
std::vector<std::set<i32>> lives(N);
for (auto i: range(0, M)) {
std::cin >> B[i] >> P[i];
lives[B[i]].insert(P[i]);
}
std::priority_queue<std::pair<i32, i32>, std::vector<std::pair<i32, i32>>, std::greater<std::pair<i32, i32>>> que;
std::vector<i32> dist(N, inf32);
dist[B[0]] = 0;
que.emplace(dist[B[0]], B[0]);
while (!que.empty()) {
const auto [d, u] = que.top();
que.pop();
if (d > dist[u]) {
continue;
}
for (auto p: lives[u]) {
{
i32 cost = 1;
i32 v = u - p;
while (v >= 0) {
if (chmin(dist[v], dist[u] + cost)) {
que.emplace(dist[v], v);
// lives[v].erase(p);
}
++cost;
v -= p;
}
}
{
i32 cost = 1;
i32 v = u + p;
while (v < N) {
if (chmin(dist[v], dist[u] + cost)) {
que.emplace(dist[v], v);
// lives[v].erase(p);
}
++cost;
v += p;
}
}
}
}
std::cout << (dist[B[1]] == inf32 ? -1 : dist[B[1]]) << '\n';
return 0;
}
