#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
const int MAXN = 3e4 + 1, inf = INT_MAX / 2;
int n, m, b[2], d[MAXN], sz = 0;
bool wasmn[MAXN];
vector <int> ps[MAXN];
priority_queue <pair<int,int>> q;
inline void dij(){
for(int i = 0; i < n; i++) d[i] = inf;
d[b[0]] = 0;
q.push({0, b[0]});
while(!q.empty()){
while(!q.empty() && wasmn[q.top().second]) q.pop();
if(q.empty()) return;
int u = q.top().second; q.pop();
wasmn[u] = true;
if(u == b[1] || d[u] >= inf) return;
for(int k: ps[u]){
for(int i = u % k; i < n; i += k)
if(d[i] > d[u] + (abs(u - i) / k)){
d[i] = d[u] + (abs(u - i) / k);
q.push({d[i], i});
}
}
}
return;
}
int main()
{
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> n >> m;
// scanf("%d", &n); scanf("%d", &m);
int bb, pp;
for(int i = 0; i < m; i++){
cin >> bb >> pp;
// scanf("%d", &bb); scanf("%d", &pp);
ps[bb].push_back({pp});
if(i == 0) b[0] = bb;
if(i == 1) b[1] = bb;
}
dij();
cout << ((d[b[1]] >= inf)?-1:d[b[1]]);
// if(d[b[1]] >= inf) printf("%d", -1);
// else printf("%d", d[b[1]]);
return 0;
}
