#include <stdio.h>

int main(void){
    int a,b,c,d,i,j,count;
    scanf("%d %d %d %d",&a,&b,&c,&d);
    for(i=a;i<=b;i++){
        for(j=c;j<=d;j++){
            if(i!=j){
                count+=1;
            }
        }
    }
    printf("%d\n",count);
    return 0;
}