#include <iostream>
#include <iomanip>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <cassert>
#include <string>
#include <set>
#include <map>
#include <random>
#include <bitset>
#include <string>
#include <unordered_set>
#include <unordered_map>
#include <deque>
#include <queue>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
using namespace std;
using ll = long long;
using ul = unsigned long long;
using ld = long double;
const int N = 30001, M = 9000000;
map<pair<short, short>, vector<short>> li;
vector<pair<int, bool>> g[M];
int d[M], used[M];
void adde(int u, int v, int w) {
g[u].push_back({v, w});
}
int main() {
ios_base::sync_with_stdio(0); cin.tie(0);
int n, m, cb, cp, s, t;
cin >> n >> m;
rep(i, m) {
cin >> cb >> cp;
if (i == 0)
s = cb;
else if (i == 1)
t = cb;
li[{cp, cb % cp}].push_back(cb);
}
int cs = n;
for (auto &pp : li) {
auto p = pp.first;
int ck = (n - p.second + p.first - 1) / p.first;
rep(i, ck - 1) {
adde(cs + i, cs + i + 1, 1);
adde(cs + i + 1, cs + i, 1);
}
rep(i, ck)
adde(cs + i, p.second + i * p.first, 0);
for (int i : pp.second)
adde(i, cs + (i - p.second) / p.first, 0);
cs += ck;
}
deque<int> q {s};
rep(i, cs)
d[i] = M;
d[s] = 0;
while (!q.empty()) {
int v = q.front();
q.pop_front();
if (used[v])
continue;
used[v] = 1;
for (auto &p : g[v]) {
if (p.second) {
if (d[p.first] > d[v] + 1) {
d[p.first] = d[v] + 1;
q.push_back(p.first);
}
}
else if (d[p.first] > d[v]) {
d[p.first] = d[v];
q.push_front(p.first);
}
}
}
if (d[t] == M)
cout << -1;
else
cout << d[t];
}
