#include <cstdio>
#define max(a,b) (a>b ? a:b)
#define min(a,b) (a<b ? a:b)
int main(){
    int A,B,C,D;
    scanf("%d %d %d %d",&A,&B,&C,&D);
    printf("%d\n",(B-A+1)*(D-C+1)-max(min(B,D)-max(A,C)+1,0));
    return 0;
}