#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define pb push_back
#define ff first
#define ss second
const int inf = 1e9;
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n, m; cin>>n>>m;
vector<int> x(m + 1), y(m + 1);
for (int i = 1; i <= m; i++){
cin>>x[i]>>y[i];
x[i]++;
}
vector<int> d[n + 1];
for (int i = 1; i <= m; i++) d[x[i]].pb(y[i]);
priority_queue<pii, vector<pii>, greater<pii>> pq; pq.push({0, x[1]});
vector<int> out(n + 1, inf); out[x[1]] = 0;
while (!pq.empty()){
auto [D, v] = pq.top(); pq.pop();
for (int j: d[v]){
int f = v + j, cc = 1;
while (f <= n){
if (out[f] > (D + cc)){
out[f] = D + cc;
pq.push({out[f], f});
}
f += j; cc++;
}
f = v - j; cc = 1;
while (f > 0){
if (out[f] > (D + cc)){
out[f] = D + cc;
pq.push({out[f], f});
}
f -= j; cc++;
}
}
}
cout<<((out[x[2]] == inf) ? -1 : out[x[2]])<<"\n";
}
