import java.util.*;
public class Main {
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);
        long a = s.nextLong();
        long b = s.nextLong();
        long c = s.nextLong();
        long k = s.nextLong();
        long ans = 0;
        if(k >= (a+b)) {
            ans = a-(k-(a+b));
        } else if(k >= a) {
            ans = a;
        } else {
            ans = k;
        }
        System.out.println(ans);
    }
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz