import java.util.Scanner;

class Main{
  public static void main(String args[]){
    Scanner scan;
    int A;
    int B;
    int C;
    int K;
    int Aa;
    int Bb;
    int Cc;

    scan = new Scanner(System.in);
    A = scan.nextInt();
    B = scan.nextInt();
    C = scan.nextInt();
    K = scan.nextInt();

    if (K >= A){
        Aa = A;
        K = K - Aa;
        if (K == 0){
            System.out.println (1*Aa);
        }else{
            if ( K >= B){
                Bb = B;
                K = K - Bb;
                if (K == 0){
                    System.out.println(1*Aa + 0*Bb);
                }else {
                    if (K >= C){
                        Cc = C;
                        K = K - Cc ;
                        if(K == 0){
                            System.out.println(1*Aa + 0*Bb + (-1*Cc));
                        }else{
                            System.out.println(1*Aa + 0*K);
                        }
                        }else{
                            System.out.println(1*Aa + 0*Bb + (-1*K));
                        }
                    }
                }
                }
    }else{
        System.out.println(1*K);
        }

    }
}
/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