#include<bits/stdc++.h>
using namespace std;
int n, m, i, j, k, A[30010], a, b, w, s, e, B[30010][2];
vector<pair<int, int> >adj[30010];
map<int, vector<int> >mp[30010];
map<int, vector<int> >::iterator it;
priority_queue<pair<int, int>, vector<pair<int, int> >, greater<pair<int, int> > >pq;
main()
{
ios_base::sync_with_stdio(!cin.tie(NULL));
for(cin>>n>>m;i++<m;)
{
cin>>B[i][0]>>B[i][1];
mp[B[i][1]][B[i][0]%B[i][1]].push_back(B[i][0]);
}
for(i=0;i++<30000;)
{
for(it=mp[i].begin();it!=mp[i].end();it++)
{
vector<int>v=it->second;
for(k=0;k<v.size();k++)
{
a=1;if(k==0)b=0;else b=max(0, v[k-1]);
for(j=v[k]-i;j>=b;j-=i,a++)
{
adj[v[k]].push_back({j, a});
}
a=1;if(k+1==v.size())b=n-1;else b=min(n-1, v[k+1]);
for(j=v[k]+i;j<=b;j+=i,a++)
{
adj[v[k]].push_back({j, a});
}
}
}
}
s=B[1][0];e=B[2][0];
for(i=0;i<n;i++)A[i]=1e9;
pq.push({0, s});
while(pq.size())
{
pair<int, int>p=pq.top();pq.pop();
a=p.second;b=p.first;
if(A[a]<1e9)continue;
A[a]=b;
for(i=0;i<adj[a].size();i++)
{
pq.push({A[a]+adj[a][i].second, adj[a][i].first});
}
}
if(A[e]==1e9)A[e]=-1;cout<<A[e];
}
