#include <iostream>
using namespace std;
int main(){
    int a,b,c,d;cin>>a>>b>>c>>d;
    int ans = (b-a+1)*(d-c+1);
    for(int i = 1; 100 >= i; i++){
        if(a <= i && i <= b && c <= i && i <= d)ans--;
    }
    cout << ans << endl;
}