import java.util.*;
import java.io.*;
import java.lang.*;

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();
        long ans = 0;
        if (k<=a){
            ans = k;
        }
        else if(k<=(a+b)){
            ans = a;
        }
        else if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        }
        System.out.println(ans);
    }
} import java.util.*; util.* java import java.io.*; io.* java import java.lang.*; lang.* 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();
        long ans = 0;
        if (k<=a){
            ans = k;
        }
        else if(k<=(a+b)){
            ans = a;
        }
        else if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        }
        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();
        long ans = 0;
        if (k<=a){
            ans = k;
        }
        else if(k<=(a+b)){
            ans = a;
        }
        else if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        }
        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();
        long ans = 0;
        if (k<=a){
            ans = k;
        }
        else if(k<=(a+b)){
            ans = a;
        }
        else if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        }
        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 long ans = 0; ans 0 if (k<=a){
            ans = k;
        }
        else if(k<=(a+b)){
            ans = a;
        }
        else if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        } k<=a k a {
            ans = k;
        } ans = k ans k if(k<=(a+b)){
            ans = a;
        }
        else if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        } k<=(a+b) k (a+b) a b {
            ans = a;
        } ans = a ans a if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        } k>(a+b) k (a+b) a b {
            ans = a + ((k-(a+b))*-1);
        } ans = a + ((k-(a+b))*-1) ans a + ((k-(a+b))*-1) a ((k-(a+b))*-1) (k-(a+b)) k (a+b) a b -1 1 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();
        long ans = 0;
        if (k<=a){
            ans = k;
        }
        else if(k<=(a+b)){
            ans = a;
        }
        else if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        }
        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();
        long ans = 0;
        if (k<=a){
            ans = k;
        }
        else if(k<=(a+b)){
            ans = a;
        }
        else if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        }
        System.out.println(ans);
    }
} Main
import java.util.*;
import java.io.*;
import java.lang.*;

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();
        long ans = 0;
        if (k<=a){
            ans = k;
        }
        else if(k<=(a+b)){
            ans = a;
        }
        else if (k>(a+b)){
            ans = a + ((k-(a+b))*-1);
        }
        System.out.println(ans);
    }
}