#include<stdio.h>
int a, b, c, d;
int main() {
    int pattern = 0;
    int i,j;
    scanf("%d %d %d %d", &a, &b, &c, &d);
    for(i = a;i <= b;i++) {
        for(j = c;j <= d;j++) {
            if(i != j)
            pattern++;
        }
    }
    printf("%d\n", pattern);
    return (0);
}