二分！！！
AC代码例如以下：
#include<iostream>
#include<cstring>
#include<cstdio>
#define ll long long
using namespace std;
ll n,m,k;
ll work(ll a)
{
ll i,j;
ll ans=0;
for(i=1;i<=n;i++)
{
j=a/i;
if(j>m)
j=m;
ans+=j;
}
return ans;
}
int main()
{
scanf("%I64d%I64d%I64d",&n,&m,&k);
ll l,r,mid;
l=1;
r=n*m+1;
while(l<r)
{
mid=(l+r)/2;
//cout<<mid<<"!!!!!!!!!!!!!!!!!!!!!!"<<work(mid)<<endl;
if(work(mid)<k) l=mid+1;
else r=mid;
}
cout<<r<<endl;
return 0;
}