import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        try (BufferedReader br = new BufferedReader(new InputStreamReader(System.in))) {
            
            String[] tmp = br.readLine().split(" ");
            int a = Integer.parseInt(tmp[0]);
            int b = Integer.parseInt(tmp[1]);
            int c = Integer.parseInt(tmp[2]);
            int k = Integer.parseInt(tmp[3]);
            
            br.close();
            
            PrintWriter pw = new PrintWriter(System.out);
            
            if (k <= a) {
                System.out.println(k);
            } else if (k <= a + b) {
                System.out.println(a);
            } else {
                System.out.println(a - (k - a - b));
            }
            
            pw.flush();
            
        }
    }
}
/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