#include<stdio.h>

int main(void){
    
    int A,B,C,D;
    scanf("%d%d%d%d",&A,&B,&C,&D);

    int n=0;
    
    if(A<=C&&C<=B){
        if(D<=B)n=D-C+1;
        if(B<=D)n=B-C+1;}
    else if(C<A&&A<D){
        if(B<=D)n=B-A+1;
        if(D<=B)n=D-A+1;}
    
    printf("%d\n",(B-A+1)*(D-C+1)-n);
    
    return 0;
}