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

int H,W;
int h,w;

int main(){

    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    cin>>H>>W>>h>>w;
    cout<<(H-h)*(W-w)<<endl;

    return 0;
}

Measured memory_bytes: 256