#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ar array
const int mxN = 3e4+10, M = 1e9+7;
int n, m, b[mxN], p[mxN], dis[mxN];
vector<ar<int, 2>> adj[mxN];
set<ar<int, 2>> s;
void dijkstra(){
priority_queue<ar<int, 2>, vector<ar<int, 2>>, greater<ar<int, 2>>> pq;
memset(dis, 0x3f, sizeof(dis));
pq.push({dis[b[0]] = 0, b[0]});
while(pq.size()){
ar<int, 2> cur = pq.top();
pq.pop();
int v = cur[1];
for(ar<int, 2> u : adj[v]){
if(dis[u[0]] > dis[v] + u[1]){
dis[u[0]] = dis[v] + u[1];
pq.push({dis[u[0]], u[0]});
}
}
}
}
int main(){
cin >> n >> m;
for(int i = 0; i<m; ++i){
cin >> b[i] >> p[i];
s.insert({b[i], p[i]});
}
for(auto v : s){
for(int h = v[0]+v[1]; h<n; h+=v[1]){
adj[v[0]].push_back({h, (h-v[0])/v[1]});
}
for(int h = v[0]-v[1]; h>=0; h-=v[1]){
adj[v[0]].push_back({h, (v[0]-h)/v[1]});
}
}
dijkstra();
cout << (dis[b[1]] >= 1e9?-1:dis[b[1]]) << "\n";
}
