import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt(), b = sc.nextInt(), c = sc.nextInt() , k = sc.nextInt();
        String s = "a";
        int result = 0;
        
        int minA =  Math.min(a, k);
        result += minA;
        
        k -= minA;
        if (k < 0) {
            System.out.println(result);
            return;
        }
        
        k -= b;
        if (k < 0) {
            System.out.println(result);
            return;
        }

        int minC =  Math.min(c, k);
        result -= minC;

        System.out.println(result);
    }
}
/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