#include <iostream>
#include<vector>
const int inf = 1e9;
using namespace std;
class doge {
public:
int i, p;
doge(int x, int y) { i = x, p = y; }
};
int ans[2005];
vector<doge> d;
int32_t main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
int n, m; cin >> n >> m;
for (int i = 0; i <= n; ++i)
ans[i] = inf;
for (int i = 0; i < m; ++i) {
int x, y; cin >> x >> y;
if (i == 1) ans[x] = 0;
doge c(x, y); d.push_back(c);
}
for (int i = 0; i < m; ++i)
if (!(abs(d[i].i - d[1].i) % d[i].p))
ans[d[i].i] = abs(d[i].i - d[1].i) / d[i].p;
bool c = 1;
for (int j = 0; ((j < n) && c); ++j) {
c = 0;
for (auto x : d) {
for (int i = x.i, val = 0; i >= 0; i -= x.p, ++val)
if (ans[x.i] > (ans[i] + val)) ans[x.i] = ans[i] + val, c = 1;
for (int i = x.i, val = 0; i < n; i += x.p, ++val)
if (ans[x.i] > (ans[i] + val)) ans[x.i] = ans[i] + val, c = 1;
}
}
cout << ((ans[d[0].i] == inf) ? -1 : ans[d[0].i]);
}
