import java.util.*;
public class Main{
	public static void main(String[] args)
	{
		Scanner in=new Scanner(System.in);
		int a=in.nextInt(),b=in.nextInt(),c=in.nextInt(),k=in.nextInt();
		int ans=0;
		if(a>=k)
		{
			System.out.println(k);
		}
		else 
		{
			if(a+b>=k)
			{
				System.out.println(a);
			}
			else
			{
				int x=a-k+a+b;
				System.out.println(x);
			}
		}
	}
}
/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