原来n×m的方格，涂去r行还剩(n-r)×m个，再把c列涂去，答案为(n-r)×(m-c)

#include<cstdio>
using namespace std;
int main(){
    long long n,m,r,c;
    while(~scanf("%lld %lld %lld %lld",&n,&m,&r,&c))
        printf("%lld\n",(n-r)*(m-c));
}