#include<stdio.h>
#include<string.h>
#include<stdlib.h>


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