#include<stdio.h>
int main(){
    int A,B,C,D,i,j,count=0;
    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++;
                break;
            }
        }
    }
    printf("%d",(B-A+1)*(D-C+1)-count);
}