#include <iostream>
#include <stdio.h>
using namespace std;
int factorial(int n){
int i;
if(n==1)
    return n=1;
else
    n*=factorial(n-1);
return n;
}
int main()
{   int n,r,n2,r2,result,result2;
    scanf("%d %d %d %d",&n,&r,&n2,&r2);
    int b1=r-n+1;
    int b2=r2-n2+1;
    result=factorial(b1)/(factorial(1)*factorial(b1-1));
result2=factorial(b2)/(factorial(1)*factorial(b2-1));
int total=result*result2;
    int beda=(r2-n);
    int beda2=r-n2;
    if(beda<0)
        beda*=-1;
    if(beda2<0)
        beda2*=-1;

    int sama=beda2-beda;
    if(sama<0)
        sama*=-1;
    cout<<total-sama;


    return 0;
}