#include <iostream>

using namespace std;

int main()
{
    int y, x, k, n, a;
    cin >> y >> x; //nyusziig km   malackaig km
    cin >> k >> n; //kezdes mez    kapott mez
    if (y>k)
    {
        a=0;
    }
    else
    {
        a=k-y;
    }
    if (n+a>=x-y)
    {
        a=(n+a)-(x-y);
    }
    else    //(n+a<x-y)
    {
        a=0;
    }
    cout << a;
    return 0;
}