#include<bits/stdc++.h>
using namespace std;
const int inf = 2e9;
int main()
{
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int n, m;
cin >> n >> m;
pair<int, int> a[m];
for(int i = 0; i < m; i++)
{
cin >> a[i].first >> a[i].second;
}
int s = a[0].first, t = a[1].first, adj[n];
fill(adj, adj+n, m);
sort(a, a+m);
for(int i = 0; i < m; i++)
{
if(!i)
{
adj[a[i].first] = i;
}
else if(a[i].first != a[i-1].first)
{
adj[a[i].first] = i;
}
}
int d[n];
fill(d, d+n, inf);
priority_queue<pair<int, int>> pq;
pq.push({d[s]=0, s});
while(pq.size())
{
int v = pq.top().second;
pq.pop();
for(int i = adj[v]; ((i < m)&&(a[i].first == v)); i++)
{
int j = a[i].second, x = 0;
while((v+(x*j) < n) || (v-(x*j) >= 0))
{
if((v+(x*j) < n) && (d[v+(x*j)] > d[v]+x))
{
pq.push({d[v+(x*j)] = d[v]+x, v+(x*j)});
}
if((v-(x*j) >= 0) && (d[v-(x*j)] > d[v]+x))
{
pq.push({d[v-(x*j)] = d[v]+x, v-(x*j)});
}
x++;
}
}
}
if(d[t] != inf)
{
cout << d[t] << '\n';
}
else
{
cout << "-1\n";
}
}
