#include <bits/stdc++.h>
#define pb push_back
#define pf push_front
using namespace std;
#define F first
#define S second
typedef long long ll;
#define pii pair <int, int>
#define pll pair <ll, ll>
typedef long double ld;
const ll N = 3 * 1e4 + 100, M = 4096 + 10, len = 21, inf = 1e18;
const ll mod = 1e9 + 7;
ll um(ll a, ll b){
return (1LL * a * b % mod + mod) % mod;
}
ll d[N], x[N], p[N];
vector <pll> q[N];
vector <ll> vec[N];
int main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
ll n, m;
cin >> n >> m;
for(ll i = 0; i < m; i++){
cin >> x[i] >> p[i];
vec[x[i]].pb(p[i]);
}
for(ll i = 0; i < n; i++){
sort(vec[i].begin(), vec[i].end());
ll c = unique(vec[i].begin(), vec[i].end()) - vec[i].begin();
for(ll j = 0; j < c; j++){
for(ll k = i + vec[i][j];; k += vec[i][j]){
if(k >= n) break;
q[i].pb({k, (k - i) / vec[i][j]});
}
for(ll k = i - vec[i][j];; k -= vec[i][j]){
if(k < 0) break;
q[i].pb({k, (i - k) / vec[i][j]});
}
}
}
for(ll i = 0; i < n; i++){
d[i] = LLONG_MAX;
}
d[x[0]] = 0;
set <pll> st;
st.insert({d[x[0]], x[0]});
while((ll)st.size() != 0){
ll v = st.begin()->S;
st.erase(st.begin());
for(auto u : q[v]){
if(d[u.F] > d[v] + u.S){
st.erase({d[u.F], u.F});
d[u.F] = d[v] + u.S;
st.insert({d[u.F], u.F});
}
}
}
if(d[x[1]] ==LLONG_MAX) cout << -1;
else cout << d[x[1]];
return 0;
}
