整除分开优化可以解决n，m到达1e9up的数据量

#include <bits/stdc++.h>
#define int long long
using namespace std;
int n, m;
int k, l, r, mid, sum;
inline int min(int a, int b)
{
    return a < b ? a : b;
}
signed main()
{
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	cin >> n >> m >> k;
	l = 1;
	r = k;
	while(l < r)
	{
		mid = l + r >> 1;
		sum = 0;
		for(int i = 1,r1; i <= min(n,mid); i=r1+1)
		{	
			r1 = min(mid/(mid/i),n);
			sum += min(m, mid / i) * (r1 - i + 1);
            //if(sum >= k) break;
		}
		if(sum >= k)
		{
			r = mid;
		}
		else
		{
			l = mid + 1;
		}
	}
	cout << l;
}