#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
const int MAXN = 3e4 + 5, inf = INT_MAX / 2;
int n, m, b[MAXN], p[MAXN], d[MAXN], cnt[MAXN], sz = 0;
vector <pair<int, int>> adj[MAXN];
set <pair<int,int>> all;
void dij(){
set <pair<int,int>> s;
for(int i = 0; i < n; i++) d[i] = inf;
d[b[0]] = 0;
s.insert({0,b[0]});
while(!s.empty()){
int v = s.begin()->second;
s.erase(*s.begin());
for(auto i: adj[v]){
int u = i.first, w = i.second;
if(d[u] > d[v] + w){
s.erase({d[u], u});
d[u] = d[v] + w;
s.insert({d[u], u});
}
}
}
return;
}
int main()
{
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> n >> m;
for(int i = 0; i < m; i++){
cin >> b[i] >> p[i];
cnt[b[i]]++;
all.insert({b[i], p[i]});
}
sz = (int)all.size();
for(int i = 0; i < sz; i++){
int bi = all.begin()->first, pi = all.begin()->second;
all.erase(*all.begin());
for(int j = bi % pi; j < n; j += pi)
if(j != bi && cnt[j])
adj[bi].push_back({j, abs(bi - j) / pi});
}
dij();
cout << ((d[b[1]] >= inf)?-1:d[b[1]]) << endl;
return 0;
}
