#include <iostream>
using namespace std;

int main()
{
    int x, y;
    cin >> y >> x;
    int k, n;
    cin >> k >> n;
    int ni=0;
    int mi=0;
   
    if((k-y)>0) ni=k-y;
    if(((ni+n)-(x-y)>0)) mi=((ni+n)-(x-y));
    cout << mi;


    return 0;
}