#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#define fi first
#define se second
#define ll long long
using namespace std;
const int N = 1e5 + 7;
const int mod = 1e9 + 7;
int n,m;
int a[N];
int b[N];
int d[N];
int h[N];
vector < pair < int , int > > v[N];
void solve()
{
cin >> n >> m;
for( int i = 0; i < m; i++ ){
cin >> a[i] >> b[i];
h[a[i]] = 1;
}
for( int i = 0; i <= n; i++ )d[i] = 1e9;
for( int i = 0; i < m; i++ ){
int f = a[i];
int cnt = 1;
while( f + b[i] <= n ){
f += b[i];
v[a[i]].push_back({ f , cnt });
cnt++;
}
f = a[i];
cnt = 1;
while( f - b[i] >= 0 ){
f -= b[i];
v[a[i]].push_back({ f , cnt });
cnt++;
}
}
d[a[0]] = 0;
set < pair < int , int > > q;
q.insert({0 , a[0]});
while( !q.empty() ){
int x = q.begin()->se;
q.erase(q.begin());
for( auto y : v[x] ){
if( d[x] + y.se < d[y.fi] ){
q.erase({d[y.fi] , y.fi});
d[y.fi] = d[x] + y.se;
q.insert({ d[y.fi] , y.fi });
}
}
}
if( d[a[1]] == 1e9 )d[a[1]] = -1;
cout << d[a[1]];
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
//freopen( "input.txt" , "r" , stdin );
//freopen( "output.txt" , "w" , stdout );
int t = 1;//cin >> t;
while( t-- ){
solve();
}
}
