import java.util.Scanner;
public class Main{
    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);

        Long A = scanner.nextLong();
        Long B = scanner.nextLong();
        Long C = scanner.nextLong();
        Long K = scanner.nextLong();
        Long count = 0L;
        if(K >= A){
            K = K - A;
            count += A;
        }else{
            count += K;
            K = 0L;
        }
        K = K - B;
        if(K > 0){
            count = count - K;
        }

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