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();
        sc.close();
        if(a > k){
            System.out.println(k);
        }else if(a + b > k){
            System.out.println(a);
        }else{
            System.out.println(2 * a - k + b);
        }
    }
} 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();
        sc.close();
        if(a > k){
            System.out.println(k);
        }else if(a + b > k){
            System.out.println(a);
        }else{
            System.out.println(2 * a - k + b);
        }
    }
} 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();
        sc.close();
        if(a > k){
            System.out.println(k);
        }else if(a + b > k){
            System.out.println(a);
        }else{
            System.out.println(2 * a - k + b);
        }
    } main {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int k = sc.nextInt();
        sc.close();
        if(a > k){
            System.out.println(k);
        }else if(a + b > k){
            System.out.println(a);
        }else{
            System.out.println(2 * a - k + b);
        }
    } 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 sc.close() sc.close sc if(a > k){
            System.out.println(k);
        }else if(a + b > k){
            System.out.println(a);
        }else{
            System.out.println(2 * a - k + b);
        } a > k a k {
            System.out.println(k);
        } System.out.println(k) System.out.println System.out System System.out k if(a + b > k){
            System.out.println(a);
        }else{
            System.out.println(2 * a - k + b);
        } a + b > k a + b a b k {
            System.out.println(a);
        } System.out.println(a) System.out.println System.out System System.out a {
            System.out.println(2 * a - k + b);
        } System.out.println(2 * a - k + b) System.out.println System.out System System.out 2 * a - k + b 2 * a - k 2 * a 2 a k b 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();
        sc.close();
        if(a > k){
            System.out.println(k);
        }else if(a + b > k){
            System.out.println(a);
        }else{
            System.out.println(2 * a - k + b);
        }
    }
} 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();
        sc.close();
        if(a > k){
            System.out.println(k);
        }else if(a + b > k){
            System.out.println(a);
        }else{
            System.out.println(2 * a - k + b);
        }
    }
} 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();
        sc.close();
        if(a > k){
            System.out.println(k);
        }else if(a + b > k){
            System.out.println(a);
        }else{
            System.out.println(2 * a - k + b);
        }
    }
}