#include <bits/stdc++.h>
#define ll long long
#define f first
#define s second
#define pii pair<int,int>
#define pll pair<ll,ll>
#define pb push_back
#define epb emplace_back
#define ull unsigned ll
using namespace std;
int main(){
int n; cin >> n;
int m; cin >> m;
vector <vector <int> > g(n + 1);
int b[m], p[m];
for(int i = 0; i < m; i++){
cin >> b[i] >> p[i];
g[b[i]].pb(i);
}
int cur = 0;
bool used[n + 1];
int d[n + 1];
fill(used, used + n + 1, false);
fill(d, d + n, 1e9);
d[b[0]] = 0;
cur = b[0];
for(int i = 1; i <= n; i++){
if(cur == -1) continue;
used[cur] = true;
int mn = 1e9, mni = -1;
for(int to : g[cur]){
for(int j = 0; j < n; j++){
if(used[j]) continue;
int v = abs(cur - j);
if(v % p[to] == 0){
v /= p[to];
d[j] = min(d[j], d[cur] + v);
}
}
}
for(int j = 0; j < n; j++){
if(!used[j] && mn > d[j])
mn = d[j], mni = j;
}
cur = mni;
}
if(d[b[1]] == 1e9)
d[b[1]] = -1;
cout << d[b[1]];
}
