/*
Solution for APIO 2015 - Skyscrapper
Tags :
*/
#include <bits/stdc++.h>
using namespace std;
#pragma GCC target ("avx2")
#pragma GCC optimization ("Ofast")
#pragma GCC optimization ("unroll-loops")
typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> ii;
typedef vector<ii> vii;
typedef long double ld;
typedef pair<ll, ll> pll;
#define FOR(i, a, b) for(int i = a; i < b; i++)
#define ROF(i, a, b) for(int i = a; i >= b; i--)
#define ms memset
#define pb push_back
#define fi first
#define se second
ll MOD = 1000000007;
ll power(ll base, ll n){
if (n == 0) return 1;
if (n == 1) return base;
ll halfn = power(base, n/2);
if (n % 2 == 0) return (halfn * halfn) % MOD;
return (((halfn * halfn) % MOD) * base) % MOD;
}
ll inverse(ll n){
return power(n, MOD-2);
}
ll add(ll a, ll b){
return (a+b) % MOD;
}
ll mul(ll a, ll b){
a %= MOD;
return (a*b) % MOD;
}
ll gcd(ll a, ll b){
if (a == 1) return 1;
if (a == 0) return b;
return gcd(b%a, a);
}
void solve(){
// O(nm * log m) dijkstra's algo
int n, m; cin >> n>> m;
vi doge[n];
int dest; int start;
FOR(i,0,m){
int b, p; cin >> b >> p;
doge[b].pb(p);
if (i==0) start = b;
if (i==1) dest = b;
}
vi dist(n,MOD);
dist[start] = 0;
set<ii> st;
st.insert({dist[start], start});
while (!st.empty()){
ii node = *st.begin();
st.erase(node);
int u = node.se;
for (int p : doge[u]){
int cur = u+p; int cnt = 1;
while (cur<n){
if (!doge[cur].empty()){
if (dist[cur] <= dist[u] + cnt){
cur += p; cnt++; continue;
}
if (dist[cur] != MOD){
st.erase({dist[cur], cur});
}
dist[cur] = dist[u] + cnt;
st.insert({dist[cur], cur});
}
cur += p; cnt++;
}
cur = u-p; cnt = 1;
while (cur >= 0){
if (!doge[cur].empty()){
if (dist[cur] <= dist[u] + cnt){
cur -= p; cnt++; continue;
}
if (dist[cur] != MOD){
st.erase({dist[cur], cur});
}
dist[cur] = dist[u] + cnt;
st.insert({dist[cur], cur});
}
cur -= p; cnt++;
}
}
}
if (dist[dest] == MOD){
cout << -1;
return;
}
cout << dist[dest];
}
int main() {
ios::sync_with_stdio(false);
int TC = 1;
//cin >> TC;
FOR(i, 1, TC+1){
//cout << "Case #" << i << ": ";
solve();
}
}
