#include <bits/stdc++.h>
using namespace std;
struct pii {
int x, i;
bool operator<(const pii b) const { return x>b.x; }
};
int main() {
ios_base::sync_with_stdio(false);cin.tie(nullptr);
int N, M, st=0, ed=-1;
cin>>N>>M;
vector<int> D(N, INT_MAX);
vector<vector<int>> A(N);
for(int i=0; i<M; i++) {
int x, m;
cin>>x>>m;
if(i==0) st=x;
else if(i==1) ed=x;
if(max(x, N-1-x)>=m) A[x].push_back(m);
}
for(int i=0; i<N; i++) {
sort(begin(A[i]), end(A[i]));
A[i].erase(unique(begin(A[i]), end(A[i])), end(A[i]));
}
priority_queue<pii> q;
q.push({0, st}); D[st]=0;
while(q.size()) {
auto[n,x]=q.top(); q.pop();
if(D[x]!=n) continue;
if(x==ed) { cout<<n; return 0; }
for(int m:A[x]) {
for(int i=x+m, d=n+1; i<N; i+=m, d++) if(d<D[i]) {
D[i]=d, q.push({d, i});
if(binary_search(begin(A[i]), end(A[i]), m)) break;
}
for(int i=x-m, d=n+1; i>=0; i-=m, d++) if(d<D[i]) {
D[i]=d, q.push({d, i});
if(binary_search(begin(A[i]), end(A[i]), m)) break;
}
}
}
cout<<-1;
}
