import java.io.BufferedReader;
import java.io.InputStreamReader;

class Main{
	public static void main(String args[])throws Exception
	{
		BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
		String s=br.readLine();
		String str[]=s.split(" ");
		int a=Integer.parseInt(str[0]);
		int b=Integer.parseInt(str[1]);
		int c=Integer.parseInt(str[2]);
		int k=Integer.parseInt(str[3]);
		if(a>=k)
			System.out.println(k);
		else if((a+b)>=k)
			System.out.println(a);
		else
		{
			int temp=k-(a+b);
			System.out.println(a-temp);
		}
	}
}

/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