import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int k = sc.nextInt();
		
		int ans = k <= a ? k : k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c); 
		
		System.out.println(ans);
	}
} import java.util.Scanner; Scanner java.util public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int k = sc.nextInt();
		
		int ans = k <= a ? k : k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c); 
		
		System.out.println(ans);
	}
} Main public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int k = sc.nextInt();
		
		int ans = k <= a ? k : k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c); 
		
		System.out.println(ans);
	} main {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int k = sc.nextInt();
		
		int ans = k <= a ? k : k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c); 
		
		System.out.println(ans);
	} Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc int b = sc.nextInt(); b sc.nextInt() sc.nextInt sc int c = sc.nextInt(); c sc.nextInt() sc.nextInt sc int k = sc.nextInt(); k sc.nextInt() sc.nextInt sc int ans = k <= a ? k : k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c); ans k <= a ? k : k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c) k <= a k a k k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c) k <= a + b k a + b a b Math.min(k, a) Math.min Math k a a - Math.min(k - a - b, c) a Math.min(k - a - b, c) Math.min Math k - a - b k - a - b k a b c System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int k = sc.nextInt();
		
		int ans = k <= a ? k : k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c); 
		
		System.out.println(ans);
	}
} public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int k = sc.nextInt();
		
		int ans = k <= a ? k : k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c); 
		
		System.out.println(ans);
	}
} Main
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int a = sc.nextInt();
		int b = sc.nextInt();
		int c = sc.nextInt();
		int k = sc.nextInt();
		
		int ans = k <= a ? k : k <= a + b ? Math.min(k, a) : a - Math.min(k - a - b, c); 
		
		System.out.println(ans);
	}
}
