#ifndef LOCAL
#define FAST_IO
#endif
// ===== template.hpp =====
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#define OVERRIDE(a, b, c, d, ...) d
#define REP2(i, n) for (i32 i = 0; i < (i32) (n); ++i)
#define REP3(i, m, n) for (i32 i = (i32) (m); i < (i32) (n); ++i)
#define REP(...) OVERRIDE(__VA_ARGS__, REP3, REP2)(__VA_ARGS__)
#define PER(i, n) for (i32 i = (i32) (n) - 1; i >= 0; --i)
#define ALL(x) begin(x), end(x)
using namespace std;
using u32 = unsigned int;
using u64 = unsigned long long;
using u128 = __uint128_t;
using i32 = signed int;
using i64 = signed long long;
using i128 = __int128_t;
using f64 = double;
using f80 = long double;
template <typename T>
using Vec = vector<T>;
template <typename T>
bool chmin(T &x, const T &y) {
if (x > y) {
x = y;
return true;
}
return false;
}
template <typename T>
bool chmax(T &x, const T &y) {
if (x < y) {
x = y;
return true;
}
return false;
}
istream &operator>>(istream &is, i128 &x) {
i64 v;
is >> v;
x = v;
return is;
}
ostream &operator<<(ostream &os, i128 x) {
os << (i64) x;
return os;
}
istream &operator>>(istream &is, u128 &x) {
u64 v;
is >> v;
x = v;
return is;
}
ostream &operator<<(ostream &os, u128 x) {
os << (u64) x;
return os;
}
template <typename F, typename Comp = less<>>
Vec<i32> sort_index(i32 n, F f, Comp comp = Comp()) {
Vec<i32> idx(n);
iota(ALL(idx), 0);
sort(ALL(idx), [&](i32 i, i32 j) -> bool {
return comp(f(i), f(j));
});
return idx;
}
[[maybe_unused]] constexpr i32 INF = 1000000100;
[[maybe_unused]] constexpr i64 INF64 = 3000000000000000100;
#ifdef FAST_IO
struct FastIO {
FastIO() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout << fixed << setprecision(10);
}
} fast_io;
#endif
// ===== template.hpp =====
#ifdef DEBUGF
#include "cpl/template/debug.hpp"
#else
#define DBG(x) (void) 0
#endif
// ===== graph.hpp =====
#include <utility>
#include <vector>
#include <numeric>
#include <cassert>
template <typename Edge>
class Graph {
std::vector<std::vector<Edge>> edges;
public:
Graph() : edges() {}
Graph(int v) : edges(v) {
assert(v >= 0);
}
std::vector<int> add_vertices(int n) {
int v = (int) edges.size();
std::vector<int> idx(n);
std::iota(idx.begin(), idx.end(), v);
edges.resize(edges.size() + n);
return idx;
}
template <typename... T>
void add_directed_edge(int from, int to, T &&...val) {
assert(from >= 0 && from < (int) edges.size());
assert(to >= 0 && to < (int) edges.size());
edges[from].emplace_back(Edge(to, std::forward<T>(val)...));
}
template <typename... T>
void add_undirected_edge(int u, int v, const T &...val) {
assert(u >= 0 && u < (int) edges.size());
assert(v >= 0 && v < (int) edges.size());
edges[u].emplace_back(Edge(v, val...));
edges[v].emplace_back(Edge(u, val...));
}
int size() const {
return (int) edges.size();
}
const std::vector<Edge> &operator[](int v) const {
assert(v >= 0 && v < (int) edges.size());
return edges[v];
}
std::vector<Edge> &operator[](int v) {
assert(v >= 0 && v < (int) edges.size());
return edges[v];
}
};
struct UnweightedEdge {
int to;
UnweightedEdge(int t) : to(t) {}
explicit operator int() const {
return to;
}
using Weight = std::size_t;
Weight weight() const {
return 1;
}
};
template <typename T>
struct WeightedEdge {
int to;
T wt;
WeightedEdge(int t, const T &w) : to(t), wt(w) {}
explicit operator int() const {
return to;
}
using Weight = T;
Weight weight() const {
return wt;
}
};
// ===== graph.hpp =====
// ===== dijkstra.hpp =====
#include <queue>
#include <limits>
#include <functional>
template <typename T>
std::vector<T> dijkstra(const Graph<WeightedEdge<T>> &g, int s) {
std::vector<T> dist(g.size(), std::numeric_limits<T>::max());
std::priority_queue<std::pair<T, int>, std::vector<std::pair<T, int>>, std::greater<>> pq;
dist[s] = T(0);
pq.emplace(0, s);
while (!pq.empty()) {
auto [d, v] = pq.top();
pq.pop();
if (d > dist[v]) {
continue;
}
for (const WeightedEdge<T> &e : g[v]) {
if (d + e.wt < dist[e.to]) {
dist[e.to] = d + e.wt;
pq.emplace(dist[e.to], e.to);
}
}
}
return dist;
}
// ===== dijkstra.hpp =====
int main() {
i32 n, m;
cin >> n >> m;
Vec<i32> b(m), p(m);
REP(i, m) {
cin >> b[i] >> p[i];
}
i32 sqrtn = (i32) floor(sqrt(n));
Graph<WeightedEdge<i64>> g((sqrtn + 1) * n);
REP(d, 1, sqrtn + 1) {
REP(i, n - d) {
i32 v = d * n + i;
g.add_undirected_edge(v, v + d, 1);
}
REP(i, n) {
i32 v = d * n + i;
g.add_directed_edge(v, i, 0);
}
}
REP(i, m) {
if (p[i] <= sqrtn) {
g.add_undirected_edge(b[i], p[i] * n + b[i], 0);
continue;
}
i32 num = n / p[i];
if (b[i] % p[i] < n % p[i]) {
++num;
}
i32 cent = b[i] / p[i];
REP(j, num) {
i32 pos = b[i] + (j - cent) * p[i];
g.add_directed_edge(b[i], pos, abs(j - cent));
}
}
DBG(dijkstra(g, b[0]));
i64 ans = dijkstra(g, b[0])[b[1]];
if (ans == numeric_limits<i64>::max()) {
cout << -1 << '\n';
} else {
cout << ans << '\n';
}
}
