#include <bits/stdc++.h>
#define mk make_pair
#define fs first
#define sc second
using namespace std;
typedef long long ll;
typedef long double ld;
// please, read the question correctly (do you need set or multiset)???
const int N=200010; //check the limits, dummy
int a[N];
int n, m;
int main(){
	// int t;
	// while(){
	int n, m, a, b;
		scanf("%d%d%d%d",&n,&m,&a,&b);
		int ans = n*b-m*a;
		cout<<ans<<endl;
	// }
}
#include <bits/stdc++.h>

using namespace std;

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	int arr[2][2];
	for(int i =0;i<2;++i){
		for(int j = 0;j<2;++j)cin >> arr[i][j];
	}
	cout << arr[0][0]*arr[1][1] - (arr[0][1]*arr[1][0]);
}
