/******************************************************************************

                            Online Java Compiler.
                Code, Compile, Run and Debug java program online.
Write your code in this editor and press "Run" button to execute it.

*******************************************************************************/
import java.util.*;
public class Main
{
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		long a = sc.nextLong();
		long b = sc.nextLong();
		long c = sc.nextLong();
		long k = sc.nextLong();
		long sum = 0;
		if(a >= k)
		{
		    sum = sum + 1 * k;
		}
		else
		{
		    k = k - a;
		    sum = sum + 1 * a;
		}
		k = k - b;
		if(k >= 1)
		{
		    if(c >= k)
		    {
		        sum = sum + (-1) * k;
		    }
		    else
		    {
		        k = k - c;
		        sum = sum + (-1) * c;
		    }
		}
		System.out.println(sum);
    }
}
/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