//Ye Rooz Khoob Miad ???
#include<bits/stdc++.h>
#define F first
#define S second
#define pb push_back
#define all(x) x.begin(), x.end()
#define sz(x) (int) (x).size
using namespace std;
typedef long long ll;
const int N = 3e5 + 2;
const int Inf = 1'061'109'567;
set<int> In[N];
bool mark[N];
int n, m, B[N], P[N], D[N];
int main(){
ios::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
cin >> n >> m;
for(int i = 0; i < m; i++) {
cin >> B[i] >> P[i];
In[B[i]].insert(P[i]);
}
priority_queue<pair<int, int>> PQ;
memset(D, 63, sizeof D);
D[B[0]] = 0;
PQ.push({0, B[0]});
while(!PQ.empty()) {
int a = PQ.top().S;
PQ.pop();
if(mark[a]) continue;
mark[a] = true;
for(int x : In[a]) {
for(int i = a + x; i < n; i += x) {
if(D[i] > D[a] + (i - a) / x) {
D[i] = D[a] + (i - a) / x;
PQ.push({-D[i], i});
}
}
for(int i = a - x; i >= 0; i -= x) {
if(D[i] > D[a] + (a - i) / x) {
D[i] = D[a] + (a - i) / x;
PQ.push({-D[i], i});
}
}
}
}
cout << (D[B[1]] == Inf ? -1 : D[B[1]]);
return 0;
}
