#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
const int MAXN = 3e4 + 5, inf = INT_MAX / 2;
int n, m, b[MAXN], p[MAXN], d[MAXN], cnt[MAXN], sz = 0;
bool wasmn[MAXN];
vector <int> ps[MAXN];
int findmn(){
ll mn = inf, ind = -1;
for(int i = 0; i < n; i++)
if(!wasmn[i] && d[i] < mn) mn = d[i], ind = i;
return ind;
}
void dij(){
for(int i = 0; i < n; i++) d[i] = inf;
d[b[0]] = 0;
for(int h = 0; h < n; h++){
int u = findmn();
wasmn[u] = true;
if(u == b[1] || d[u] >= inf) return;
for(auto k: ps[u]){
for(int i = u % k; i < n; i += k)
if(i != u && cnt[i] && d[i] > d[u] + (abs(u - i) / k)) d[i] = d[u] + (abs(u - i) / k);
}
}
return;
}
int main()
{
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> n >> m;
for(int i = 0; i < m; i++){
cin >> b[i] >> p[i];
cnt[b[i]]++;
ps[b[i]].push_back({p[i]});
}
dij();
cout << ((d[b[1]] >= inf)?-1:d[b[1]]) << endl;
return 0;
}
