#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 x[N], p[N], d[N];
vector <pll> q[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];
}
for(ll i = 0; i < m; i++){
for(ll j = x[i] + p[i];; j += p[i]){
if(j >= n) break;
//cout << j << " ";
q[x[i]].pb({j, (j - x[i])/p[i]});
}
//cout << endl;
for(ll j = x[i] - p[i];; j -= p[i]){
if(j < 0) break;
q[x[i]].pb({j, (x[i] - j)/p[i]});
}
}
for(ll i = 0; i < n; i++){
d[i] = LLONG_MAX;
}
d[0] = 0;
set <pll> st;
st.insert({d[0], 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[1] ==LLONG_MAX) cout << -1;
else cout << d[1];
return 0;
}
