//compiledef
#include <bits/stdc++.h>
#define rep(i, n) for(int i=0;i<(n);i++)
#define per(i, n) for(int i=(n);i>0;i--)
#define repx(i, n, x) for(int i=(x);i<(n);i++)
#define xper(i, n, x) for(int i=(n);i>(x);i--)
#define pback push_back

//c++def
using namespace std;
typedef long long int ll;
typedef unsigned long long int ull;
const ll MOD = 1e9+7;
const ll MOD2 = 998244353;

int main(){
	int a, b, c, d;
	cin >> a>> b >> c >> d;
	cout << (a*d) -(b*c) << endl;
}

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

int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  cout << a * d - b * c << endl;
}