#include <iostream>
using namespace std;

int main(){
    short int a, b, c, d;
    cin >> a >> b >> c >> d;
    int p = 0;
    if(c <= b)
        p = b - c + 1;
    cout << (b - a + 1) * (d - c + 1) - p << "\n";
    
    return 0;
}