import java.io.BufferedInputStream;
import java.util.Scanner;

public class Main {
    private static Scanner sc = new Scanner(new BufferedInputStream(System.in));

    public static void main(String[] args) {
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int k = sc.nextInt();
        long sum = 0;
        if (a >= k) {
            sum += k;
            k = 0;
        } else {
            sum += a;
            k -= a;
        }
        k -= b;
        if (k > 0) {
            sum -= k;
        }
        System.out.println(sum);
    }
}
/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