#include<iostream>
using namespace std;
int main()
{
	int a,b,c,d;
	cin>>a>>b>>c>>d;
	cout<<a*d-b*c<<endl;
	return 0;
 } 
//Bismillahirrahmanirrahim

#pragma GCC optimize ("O3")
#pragma GCC target ("sse4")
#include <bits/stdc++.h>

using namespace std;

typedef long long lo; 
typedef pair< lo,lo > PII;

#define fi first
#define se second
#define mp make_pair
#define endl "\n"
#define pb push_back
#define fio() ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL)
#define FOR for(int i=1;i<=n;i++)
#define mid ((start+end)/2)
#define ort ((bas+son)/2)

const lo inf = 1000000000000000000;
const lo KOK = 100000;
const lo LOG = 30;
const lo li = 500005;
const lo mod = 1000000007;

int n,m,b[li],a[li],k,flag,t;
int cev;
string s;
vector<int> v;

int main(void){
	scanf("%d %d %d %d",&n,&m,&k,&t);
	printf("%d\n",n*t-m*k);
	return 0;
}


