#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, vis[N], j[N], dp[300001], pos[300001];
vector <int> v[N], v1[N];
void solve(int dog) {
if(abs(pos[dog] - pos[1]) % j[dog] == 0){
dp[pos[1]] = min(dp[pos[1]], dp[pos[dog]] + abs(pos[dog] - pos[1]) / j[dog]);
return;
}
if(vis[dog]) return;
vis[dog] = 1;
for(auto i : v[dog]) {
if(dp[i] < dp[pos[dog]] + abs(i - pos[dog]) / j[dog]){
dp[i] = dp[pos[dog]] + abs(i - pos[dog]) / j[dog];
for(auto k : v1[i]) {
solve(k);
}
}
}
}
int main () {
ios::sync_with_stdio(0);cin.tie(0);
cin >> n >> m;
for(int i = 0; i < n; i++) {
dp[i] = INT_MAX;
}
for(int i = 0; i < m; i++) {
int x, y;
cin >> x >> y;
int k = 1;
pos[i] = x;
v[i].pb(x);
v1[x].pb(i);
while(x - k * y > 0 || x + k * y <= n) {
if(x - k * y > 0) v[i].pb(x - k * y);
if(x + k * y <= n)v[i].pb(x + k * y);
k++;
}
j[i] = y;
}
dp[pos[0]] = 0;
solve(0);
if(dp[pos[1]] == INT_MAX) dp[pos[1]] = -1;
cout << dp[pos[1]] <<'\n';
}
