#include <iostream>
using namespace std;
int main()
{
    int tav1, tav2, adag1, adag2;
    cin >> tav1;
    cin >> tav2;
    tav2 = tav2 - tav1;
    cin >> adag1;
    cin >> adag2;
    if (adag1 - tav1 < 0) {
        if (adag2 - tav2 < 0) {
            cout << 0;
        }
        else {
            cout << adag2 - tav2;
        }
    }
    else {
        if (adag1 - tav1 + adag2 - tav2 < 0) {
            cout << 0;
        }
        else {
            cout << adag1 - tav1 + adag2 - tav2;
        }
    }
}