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

	    if(k >= b){
	        k -= b;
	    }
	    else{
	        k = 0;
	    }
	
	    if(k >= c){
	        ans -= c;
	        k -= c;
	    }
	    else{
	        ans = ans-k;
	    }
	    System.out.println(ans);
	}
}


/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