#include <stdio.h>
#include <stdlib.h>

int main(void){
    int a,b,c,d;
    int xxx;
    int ans;
    int min;
    int max;
    scanf("%d%d%d%d",&a,&b,&c,&d);
    min = (a < c)? a:c;
    max = (b < d)? d:b;
    ans = (b - a + 1)*(d - c + 1);
    xxx = ((b - a + 1) + (d - c + 1)) - (max - min + 1);
    if(xxx > 0){
        ans -= xxx;
    }
    else{
        ans = max - min +1;
    }
    printf("%d",ans);

    
}