#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn = 30000 + 10;
const int inf = 1e9;
int b[maxn], p[maxn], dp[maxn], mark[maxn];
int main(){
ios_base::sync_with_stdio(false);
int n, m;
cin >> n >> m;
for (int i = 0; i < m; i++)
cin >> b[i] >> p[i];
for (int i = 0; i < m; i++)
dp[i] = inf;
dp[0] = 0;
for (int _ = 0; _ < m; _++){
int v = -1;
for (int i = 0; i < m; i++)
if (!mark[i] and (v == -1 or dp[v] > dp[i]))
v = i;
if (dp[v] == inf)
return cout << -1 << endl, 0;
if (v == 1)
return cout << dp[1] << endl, 0;
mark[v] = 1;
for (int u = 0; u < m; u++){
if (mark[u])
continue;
if (abs(b[v] - b[u]) % p[v] == 0)
dp[u] = min(dp[u], dp[v] + abs(b[v] - b[u]) / p[v]);
}
}
}
