#include <iostream>
#include <stdio.h>
using namespace std;

int main()
{   int n,r,n2,r2,result,result2;
    scanf("%d %d %d %d",&n,&r,&n2,&r2);
    int s=r-n+1;
    int k=r2-n2+1;
    int ma=max(n,n2);
    int mi=min(r,r2);
    int tot=ma-mi;
    if(tot>0){
    cout<<s*k<<endl;
    }
    else{
        cout<<s*k-(tot+1)<<endl;
    }
    


    return 0;
}