#include "bits/stdc++.h"
using namespace std;
#define int long long
//#define endl '\n'
int mod=1e9+7;
int mod2=998244353;
const int INF=1e9;

signed main(){
  int h,w,a,b;
  cin>>h>>w>>a>>b;
  cout<<h*w-a*w-b*h+a*b<<endl;
}

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