/*
TASK:area
LANG:C++
*/

#include <cstdio>
//#include <conio.h>
#include <algorithm>
#include <iostream>
using namespace std;

typedef long long ll;
long long X1,Y1,X2,Y2;
int N;

int main()
{
    //freopen("a4.in","r",stdin);
    
    //scanf("%d",&N);
    //printf("%d\n",N);
    
    //scanf("%d%d%d%d",&X1,&Y1,&X2,&Y2);
    cin>>X1>>Y1>>X2>>Y2;
    long long S=(ll)((ll)abs(X2-X1))*((ll)abs(Y2-Y1));

    cout<<S<<"\n";
    
    //getch();
    return 0;
}
