#include <stdio.h>

int maximum(int,int);
int minimum(int,int);

int main(void)
{
    int a,b,c,d;
    int max,min,tmp,ans;
    scanf("%d%d%d%d",&a,&b,&c,&d);

    max = maximum(a,c);
    min = minimum(b,d);
    tmp = maximum(0,(min-max+1));
    ans = (b-a+1)*(d-c+1)-maximum(0,tmp);

    printf("%d\n",ans);
    return 0;
}

int maximum(int a,int b)
{
    if(a > b){
        return a;
    } else {
        return b;
    }
}

int minimum(int a,int b)
{
    if(a < b){
        return a;
    } else {
        return b;
    }
}