import java.util.*;
public class Main{
    public static void main(String args[]){
        Scanner sn = new Scanner(System.in);
        int A = sn.nextInt();
        int B = sn.nextInt();
        int C = sn.nextInt();
        int K = sn.nextInt();
        int flag = 1;
        int ans = 0;

        for(int i=0; i<A; i++){
            if(K == 0) {
                break;
            }
            ans = ans + 1;
            K = K - 1;
        }

        for(int i=0; i<B; i++){
            if(K == 0) {
                break;
            }
            K = K - 1;
        }

        for(int i=0; i<C; i++){
            if(K == 0) {
                break;
            }
            ans = ans - 1;
            K = K - 1;
        }

        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