///* __ __ __ */
///* ====== _ /| /| __ _ / | | /| | * | | | | / /| |\ | / | | * | / */
///* \- || |_| |_ / |/ | | | |_ |- | |--| /-| | | \ \ |==| |- /=| | \ | | |--| | |- */
///* || | | |_ / | |__| _| |_ \__ | | / | |__ | __| | | | \ / | | \| \__ | | | | \ */
///*
// autor :: Rick Prime
#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define ff first
#define ss second
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define int long long
#define Mp make_pair
#define double double long
typedef vector<int> vi;
typedef vector<double> vd;
typedef pair<int,int> pii;
typedef vector<pii> vii;
const int N = 1e6, inf = 2e18, mod = 1e9+7;
void solve(){
int n, m;
cin >> n >> m;
vii v(m);
for(auto &[i, j]:v)cin >> i >> j;
int dp[n];
for(int &i:dp)i = inf;
dp[v[0].ff] = 0;
for(int i = 0; i < m; i++){
for(auto [b, p]:v){
if(dp[b] == inf)continue;
for(int x = b+p, ct = 1; x < n; x += p, ct++){
dp[x] = min(dp[x], dp[b] + ct);
}for(int x = b-p, ct = 1; x >= 0; x -= p, ct++){
dp[x] = min(dp[x], dp[b] + ct);
}
}
}
cout << (dp[v[1].ff]==inf?-1:dp[v[1].ff]) << '\n';
}
signed main(){
ios_base::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);
int test = 1;
//cin >> test;
while(test--){
solve();
}
}
