#include<bits/stdc++.h>
#pragma optimize ("g",on)
#pragma GCC optimize ("inline")
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("unroll-loops")
#pragma GCC optimize ("03")
#pragma GCC target ("sse,sse2,sse3,ssse3,sse4,popcnt,abm,avx2,mmx,fma,avx,tune=native")
//01001101 01100001 01101011 01101000 01100001 01100111 01100001 01111001
using namespace std;
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define ll long long
#define pb push_back
#define sz(a) a.size()
#define nl '\n'
#define popb pop_back()
#define ld double
#define ull unsigned long long
#define ff first
#define ss second
#define fix fixed << setprecision
#define pii pair<int, int>
#define E exit (0)
#define int long long
const int inf = 1e15, N = 5e3 + 1, mod = 998244353;
double eps = 1e-6;
vector<pii> dir = {{1, 0}, {0, 1}, {-1, 0}, {0, -1}};
main() {
//freopen("F.in", "r", stdin);
//freopen("F.out", "w", stdout);
ios_base::sync_with_stdio(NULL);
cin.tie(NULL);
int n, m;
cin >> n >> m;
int b[m + 1], p[m + 1];
vector<int> h[n + 1];
for (int i = 1; i <= m; i++) {
cin >> b[i] >> p[i];
h[b[i]].pb(p[i]);
}
set<pii> s;
vector<int> d(n + 1, inf);
s.insert({0, b[1]});
d[b[1]] = 0;
while (!s.empty()) {
int u = (*s.begin()).ss;
s.erase(s.begin());
for (auto e: h[u]) {
for (int j = u; j <= n; j += e) {
if (d[j] > d[u] + (j - u) / e) {
s.erase({d[j], j});
d[j] = d[u] + (j - u) / e;
s.insert({d[j], j});
}
}
for (int j = u - e; j >= 0; j -= e) {
if (d[j] > d[u] + (u - j) / e) {
s.erase({d[j], j});
d[j] = d[u] + (u - j) / e;
s.insert({d[j], j});
}
}
}
}
if (d[b[2]] == inf) d[b[2]] = -1;
cout << d[b[2]];
}
