#include "bits/stdc++.h"
using namespace std;
#define N 30005
#define ll long long
#define ff first
#define ss second
#define pb push_back
#define SZ(v) (int)v.size()
int n, m, ans = INT_MAX, j[N], dp[300001], pos[300001];
vector <pair<int,int>> v;
void solve(int dog) {
if(abs(pos[dog] - pos[1]) % j[dog] == 0){
dp[1] = min(dp[1], dp[dog] + abs(pos[dog] - pos[1]) / j[dog]);
return;
}
for(int i = 0; i < SZ(v); i++) {
if(i == dog || abs(v[i].ff - pos[dog]) % j[dog] != 0 || dp[i] <= dp[dog] + abs(v[i].ff - pos[dog]) / j[dog]) continue;
dp[i] = dp[dog] + abs(v[i].ff - pos[dog]) / j[dog];
solve(i);
}
}
int main () {
ios::sync_with_stdio(0);cin.tie(0);
cin >> n >> m;
for(int i = 0; i < m; i++) {
dp[i] = INT_MAX;
}
for(int i = 0; i < m; i++) {
int x, y;
cin >> x >> y;
j[i] = y;
pos[i] = x;
v.pb({x,y});
}
dp[0] = 0;
solve(0);
if(dp[1] == INT_MAX) dp[1] = -1;
cout << dp[1] <<'\n';
}
