858B - Which floor - CodeForces Solution

//CF858B

#include <bits/stdc++.h>
#define int long long
using namespace std;

int n, m;
struct node
{
	int id, flr;
}a[105];

bool check(int f)
{
	for (int i = 1; i <= m; i++)
	{
		if (ceil(a[i].id * 1.0 / f) != a[i].flr)
			return false;
	}
	return true;
}

signed main()
{
	int ans = -1;
	scanf("%lld%lld", &n, &m);
	for (int i = 1; i <= m; i++)
		scanf("%lld%lld", &a[i].id, &a[i].flr);
	for (int i = 1; i <= 100; i++)
	{
		if (check(i))
		{
			int now = ceil(n * 1.0 / i);
			if (ans != -1 && now != ans)
			{
				printf("-1");
				return 0;
			}
			ans = now;
		}
	}
	printf("%lld", ans);
	return 0;
}