import 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;
        if(k <= a)
        	ans = k;
        else if(k <= (a + b))
        	ans = a;
        else {
        	ans = a - (k - (a + b));
        }
        System.out.println(ans);
    }
} import java.util.*; util.* java 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;
        if(k <= a)
        	ans = k;
        else if(k <= (a + b))
        	ans = a;
        else {
        	ans = a - (k - (a + b));
        }
        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;
        if(k <= a)
        	ans = k;
        else if(k <= (a + b))
        	ans = a;
        else {
        	ans = a - (k - (a + b));
        }
        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;
        if(k <= a)
        	ans = k;
        else if(k <= (a + b))
        	ans = a;
        else {
        	ans = a - (k - (a + b));
        }
        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; ans if(k <= a)
        	ans = k;
        else if(k <= (a + b))
        	ans = a;
        else {
        	ans = a - (k - (a + b));
        } k <= a k a ans = k ans k if(k <= (a + b))
        	ans = a;
        else {
        	ans = a - (k - (a + b));
        } k <= (a + b) k (a + b) a b ans = a ans a {
        	ans = a - (k - (a + b));
        } ans = a - (k - (a + b)) ans a - (k - (a + b)) a (k - (a + b)) k (a + b) a b 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;
        if(k <= a)
        	ans = k;
        else if(k <= (a + b))
        	ans = a;
        else {
        	ans = a - (k - (a + b));
        }
        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;
        if(k <= a)
        	ans = k;
        else if(k <= (a + b))
        	ans = a;
        else {
        	ans = a - (k - (a + b));
        }
        System.out.println(ans);
    }
} Main
import 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;
        if(k <= a)
        	ans = k;
        else if(k <= (a + b))
        	ans = a;
        else {
        	ans = a - (k - (a + b));
        }
        System.out.println(ans);
    }
}