#include <bits/stdc++.h>
#define int short
#define pii pair<int, int>
using namespace std;
short MXN = 30000;
short n, m, dis[30001], b, p, d=174, st, en, cnt, cd, cb, cp, nd, nb, np;
vector<int> g[30001];
priority_queue<pii, vector<pii>, greater<pii>> pq;
signed main() {
cin >> n >> m;
for(int i=0; i<30001; i++) {
dis[i] = 32700;
}
for(int i=0; i<m; i++) {
cin >> b >> p;
if(i == 0) st = b;
if(i == 1) en = b;
g[b].push_back(p);
}
pq.push({0, st});
while(pq.size()) {
cd = pq.top().first;
cb = pq.top().second;
pq.pop();
if(cd > dis[cb]) continue;
for(auto i: g[cb]) {
for(int j=1; cb+(i*j) < n; j++) {
nb = cb+(i*j);
if(cd+j < dis[nb]) {
dis[nb] = cd+j;
pq.push({cd+j, nb});
}
}
for(int j=1; cb-(i*j) >= 0; j++) {
nb = cb-(i*j);
if(cd+j < dis[nb]) {
dis[nb] = cd+j;
pq.push({cd+j, nb});
}
}
}
}
if(dis[en] == 32700) cout << -1;
else cout << dis[en];
return 0;
}
