#include<bits/stdc++.h>

using namespace std;

int main(){
int a =0 , b=0 , c =0 , d=0;
cin >> a>> b >> c >> d;
cout << (a*d) - (b*c) <<endl;

return 0;}

#include <bits/stdc++.h>
#define rep(i,n)    for(int i=0;i<(n);++i)
#define repA(i,k,n) for(int i=k;i<=(n);++i)
#define repD(i,k,n)	for(int i=k;i>=(n);--i)
#define endl '\n'
#define print(b)	for(auto a:b) cout<<a<<' ';
#define printN(b)   for(auto a:b) cout<<a<<endl;
#define fin    		freopen("C:\\Users\\Nazmul Rahul\\Desktop\\in.txt", "r++", stdin)
#define fout   		freopen("C:\\Users\\Nazmul Rahul\\Desktop\\out.txt", "w++", stdout)
#define fastio    	ios::sync_with_stdio(false)
#define max(a,b)	((a)>(b)?(a):(b))
#define min(a,b)	((a)<(b)?(a):(b))
#define pb    		push_back
#define pob   		pop_back
#define ll 			long long int
using namespace std;
int main() {
#ifndef ONLINE_JUDGE
	fin;
	fout;
#endif
	int ara[2][2];
	for (int i = 0; i < 2; i++) {
		for (int j = 0; j < 2; j++)
			cin >> ara[i][j];
	}
	cout << ((ara[0][0]*ara[1][1]) - (ara[0][1]*ara[1][0]));
	cout << endl;


	return 0;

}