import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class Main {
	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		StringTokenizer st = new StringTokenizer(br.readLine());
		int po = Integer.parseInt(st.nextToken());
		int z = Integer.parseInt(st.nextToken());
		int no = Integer.parseInt(st.nextToken());
		int K = Integer.parseInt(st.nextToken());
		if (K <= po) {
			System.out.println(K);
		} else if (K <= po + z) {
			System.out.println(po);
		} else {
			System.out.println(po - (K - po - z));
		}
	}
}

/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