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

	    
	    int a=scan.nextInt();
	    int b=scan.nextInt();
	    int c=scan.nextInt();
	   int k=scan.nextInt();
	   int i=0;
	   int sum=0;
	   if(k>=a)
	  { sum+=a;
		  k=k-a;
	  }
	  else
	    {sum=k;
			k=0;
		}
		if(k>=b)
		k=k-b;
		else k=0;
		if(k>=c)
		sum=sum-c;
	    else sum=sum-k;
	   
	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