import java.util.Scanner;

public class Main {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);

        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        int K = sc.nextInt();
        int point = 0;
        int diff;

        if (A >= K) {
            point = K;
        } else if ((A + B) >= K) {
            point = A;
        } else {
            diff = K - (A + B);
            point = A - diff;
        }

        System.out.print(point);
    }
}
/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