import java.util.Scanner;
//import java.util.Arrays;
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 result = 0;
		if(k >= a){
			result += a;
			if(k > a + b){
				result -= k - a - b;
			}
		}else{
			result += k;
		}
		System.out.println(result);
	}
}

/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