#include <iostream>

int main(){
    int a,b,c,d,i,j,count;
    std::cin >> a >> b >> c >> d;
    for(i=a;i<=b;i++){
        for(j=c;j<=d;j++){
            if(i!=j){
                count+=1;
            }
        }
    }
    std::cout << count << std::endl;
}