#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 a,b,c,d;
  cin>>a>>b>>c>>d;
  cout<<a*b-(c*d*(a/c)*(b/d))<<endl;
}

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