codeforces 257c

#include<stdio.h>
int main() {
  __int64 n,m,k,i,j,a,b;
  while(scanf("%I64d%I64d%I64d",&n,&m,&k)!=EOF) {
    if(n+m-2<k) {
        printf("-1\n");
        continue;
    }
    a=k+1;
    b=1;
    if(k+1>n) {
     a=n;
     b=k+2-n;
    }
    i=(n/a)*(m/b);
    a=k+1;b=1;
    if(k+1>m) {
        a=m;
        b=k+2-m;
    }
    j=(m/a)*(n/b);
    printf("%I64d\n",i>j?i:j);
  }
return 0;
}