#include <bits/stdc++.h>
//#pragma GCC optimize("Ofast")
#define fi first
#define se second
#define ll long long
using namespace std;
const int N = 1e6 + 7;
const int mod = 1e9 + 7;
int n,m;
int a[N];
int b[N];
int d[N];
int h[N];
int t[N];
set < int > s[N];
vector < pair < int , int > > v[N];
void solve()
{
cin >> n >> m;
for( int i = 0; i < m; i++ ){
cin >> a[i] >> b[i];
s[a[i]].insert(b[i]);
}
for( int i = 0; i <= n; i++ )d[i] = 1e9;
for( int i = 0; i < n; i++ ){
auto it = s[i].begin();
while( it != s[i].end() ){
int z = i , cnt = 0;
while( z + *it < n ){
z += *it , cnt++;
v[i].push_back({z , cnt});
if( s[z].find(*it) != s[z].end() )break;
}
z = i , cnt = 0;
while( z - *it >= 0 ){
z -= *it , cnt++;
v[i].push_back({z , cnt});
if( s[z].find(*it) != s[z].end() )break;
}
it++;
}
}
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();
}
}
