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