#include <bits/stdc++.h>
#define int long long
int b[30010];
int p[30010];
std::vector<std::pair<int,int>> adj[30010];
int dist[30010];
signed main() {
int n,m;
std::cin >> n >> m;
int s=0,e=1;
for(int i=0;i<m;i++){
std::cin >> b[i] >> p[i];
if(i==0)s=b[i];
else if(i==1)e=b[i];
}
for(int i=0;i<n;i++){
dist[i]=1e18;
}
std::priority_queue<std::pair<int,int>> pq;
pq.push({-0,s});
while(!pq.empty()){
int weight=-pq.top().first;
int node=pq.top().second;
pq.pop();
for(int j=b[node];j<n;j+=p[node]){
int w=(j-b[node])/p[node];
if(weight+w<dist[j]){
dist[j]=weight+w;
pq.push({-(dist[j]),j});
}
}
for(int j=b[node]-p[node];j>=0;j-=p[node]){
int w=(b[node]-j)/p[node];
if(weight+w<dist[j]){
dist[j]=weight+w;
pq.push({-(dist[j]),j});
}
}
}
if(dist[e]>=1e18)dist[e]=-1;
std::cout << dist[e];
}
