#include<stdio.h>
int a, b, c, d;
int main(void) {
    int value = 0;
    int pattern = 1;
    int i;
    scanf("%d %d %d %d", &a, &b, &c, &d);
    value = (b - a + 1) + (d - c + 1);
    
    for(i = 1;i <= value;i++) {
        pattern = pattern * i;
    }
    
    printf("%d\n", pattern);
    
    return (0);
}