import java.util.*;

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 = 0;
       
        if( k<=a ){
            ans = k;
        }else if( a<k & k<=a+b ){
            ans = a; 
        }else if( a+b<k & k<=a+b+c){
            ans = a - (k-a-b);
        }

        System.out.println( ans );

    }
} import java.util.*; util.* java 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 = 0;
       
        if( k<=a ){
            ans = k;
        }else if( a<k & k<=a+b ){
            ans = a; 
        }else if( a+b<k & k<=a+b+c){
            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 = 0;
       
        if( k<=a ){
            ans = k;
        }else if( a<k & k<=a+b ){
            ans = a; 
        }else if( a+b<k & k<=a+b+c){
            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 = 0;
       
        if( k<=a ){
            ans = k;
        }else if( a<k & k<=a+b ){
            ans = a; 
        }else if( a+b<k & k<=a+b+c){
            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 = 0; ans 0 if( k<=a ){
            ans = k;
        }else if( a<k & k<=a+b ){
            ans = a; 
        }else if( a+b<k & k<=a+b+c){
            ans = a - (k-a-b);
        } k<=a k a {
            ans = k;
        } ans = k ans k if( a<k & k<=a+b ){
            ans = a; 
        }else if( a+b<k & k<=a+b+c){
            ans = a - (k-a-b);
        } a<k & k<=a+b a<k a k k<=a+b k a+b a b {
            ans = a; 
        } ans = a ans a if( a+b<k & k<=a+b+c){
            ans = a - (k-a-b);
        } a+b<k & k<=a+b+c a+b<k a+b a b k k<=a+b+c k a+b+c a+b+c a b c {
            ans = a - (k-a-b);
        } ans = a - (k-a-b) ans a - (k-a-b) a (k-a-b) k-a-b k a b System.out.println( ans ) System.out.println System.out System System.out ans String[] args args
import java.util.*;

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 = 0;
       
        if( k<=a ){
            ans = k;
        }else if( a<k & k<=a+b ){
            ans = a; 
        }else if( a+b<k & k<=a+b+c){
            ans = a - (k-a-b);
        }

        System.out.println( ans );

    }
}