#include <iostream>
#include <algorithm>
#include <vector>
#include <random>
#include <chrono>
#include <set>
#include <map>
#include <stack>
#include <functional>
#include <iomanip>
#include <queue>
#include <cassert>
#include <complex>
#include <cstring>
#include <memory>
#include <bitset>
#include <sstream>
#include <cmath>
#include <numeric>
#include <numbers>
#include <fstream>
using namespace std;
#ifndef template
#ifndef define
#define ll long long
#define ld long double
#define pl pair<ll, ll>
#define pi pair<int, int>
#define nl cout << '\n';
#define x first
#define y second
#define cbit(x) __builtin_popcountll(x)
#define uid(a, b) uniform_int_distribution<ll>(a, b)(rng)
#define siz(x) (int)x.size()
#endif
#ifndef print
void print(size_t x) {cout << x << ' ';}
void print(int x) {cout << x << ' ';}
void print(long long x) {cout << x << ' ';}
void print(float x) {cout << x << ' ';}
void print(long double x) {cout << x << ' ';}
void print(char x) {cout << x << ' ';}
void print(const char* x) {cout << x << ' ';}
void print(bool x) {cout << x << ' ';}
void print(string &x) {cout << x << ' ';}
template<typename T, typename V>
void print(pair<T, V> &p) {print(p.x); print(p.y);}
template<typename T>
void print(vector<T> v) {for (int i = 0; i < v.size(); i++) print(v[i]);}
template<typename T>
void print(vector<vector<T>> v) {
for (int i = 0; i < v.size(); i++){
for (int j = 0; j < v[i].size(); j++)
print(v[i][j]);
nl;
}
}
template <typename T, typename... V>
void print(T t, V&&... v) {print(t); print(v...);}
#endif
#ifndef read
void read(int &x) {cin >> x;}
void read(long long &x) {cin >> x;}
void read(unsigned &x) {cin >> x;}
void read(unsigned long long &x) {cin >> x;}
void read(float &x) {cin >> x;}
void read(long double &x) {cin >> x;}
void read(char &x) {cin >> x;}
void read(string &x) {cin >> x;}
void read(bool &x) {cin >> x;}
template<typename T>
void read(vector<T> &v) {
for (int i = 0; i < v.size(); i++)
read(v[i]);
}
template <typename T, typename... V>
void read(T &t, V&... v) {read(t); read(v...);}
#endif
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
template<class T> bool maxi(T& a, const T& b) {
return a < b ? a = b, 1 : 0;
}
template<class T> bool mini(T& a, const T& b) {
return a > b ? a = b, 1 : 0;
}
template<class... Args>
auto vec(size_t n, Args&&... args) {
if constexpr(sizeof...(args) == 1)
return vector(n, args...);
else
return vector(n, vec(args...));
}
#endif
using namespace std;
const int inf = 1e9;
const ll def = 1e7+1;
const ll mod = 998244353;
void solve(){
int n, m;
read(n, m);
int block = sqrt(n);
auto dp = vec(2 * n, block + 1, inf);
auto edg = vec(2 * n, block + 1, vector<pair<int, pi>>());
int s, t;
for (int i = 0; i < m; i++){
int u, x;
read(u, x);
if (i == 0)
s = u;
else if (i == 1)
t = u;
if (x <= block)
edg[u][0].push_back({0, {u + n, x}});
else{
for (int j = u; (j + x) < n; j += x)
edg[u][0].push_back({(j - u) / x + 1, {j + x, 0}});
for (int j = u; (j - x) >= 0; j -= x)
edg[u][0].push_back({(u - j) / x + 1, {j - x, 0}});
}
}
for (int i = 0; i < n; i++) for (int j = 1; j <= block; j++){
edg[i + n][j].push_back({0, {i, 0}});
int u = i + j;
if (u >= n)
continue;
edg[i + n][j].push_back({1, {u + n, j}});
edg[u + n][j].push_back({1, {i + n, j}});
}
priority_queue<pair<int, pi>, vector<pair<int, pi>>, greater<pair<int, pi>>> pq;
dp[s][0] = 0;
pq.push({0, {s, 0}});
while (pq.size()){
int dis = pq.top().x;
auto [u, x] = pq.top().y;
pq.pop();
if (dis > dp[u][x])
continue;
for (auto [w, it] : edg[u][x]){
auto [v, y] = it;
int ndis = dis + w;
if (ndis < dp[v][y]){
dp[v][y] = ndis;
pq.push({ndis, {v, y}});
}
}
}
int res = dp[t][0];
if (res == inf)
res = -1;
print(res);
}
/*
*/
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
if (ifstream("input.txt").good()){
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
}
int t;
t = 1;
while (t--){
solve();
nl;
}
}
