import java.util.*;

public class Main {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());
        int c = Integer.parseInt(sc.next());
        int k = Integer.parseInt(sc.next());
        int ans = 0;

        if (a >= k) {
                ans = k;
            } else {
                ans += a;
                k -= a;
                if (b >= k) {
                    k = 0;
                } else {
                    k -= b;
                    ans -= k;
                    }
            }
        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 = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());
        int c = Integer.parseInt(sc.next());
        int k = Integer.parseInt(sc.next());
        int ans = 0;

        if (a >= k) {
                ans = k;
            } else {
                ans += a;
                k -= a;
                if (b >= k) {
                    k = 0;
                } else {
                    k -= b;
                    ans -= k;
                    }
            }
        System.out.println(ans);
        }
} Main public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());
        int c = Integer.parseInt(sc.next());
        int k = Integer.parseInt(sc.next());
        int ans = 0;

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

        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());
        int c = Integer.parseInt(sc.next());
        int k = Integer.parseInt(sc.next());
        int ans = 0;

        if (a >= k) {
                ans = k;
            } else {
                ans += a;
                k -= a;
                if (b >= k) {
                    k = 0;
                } else {
                    k -= b;
                    ans -= k;
                    }
            }
        System.out.println(ans);
        } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = Integer.parseInt(sc.next()); a Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int b = Integer.parseInt(sc.next()); b Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int c = Integer.parseInt(sc.next()); c Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int k = Integer.parseInt(sc.next()); k Integer.parseInt(sc.next()) Integer.parseInt Integer sc.next() sc.next sc int ans = 0; ans 0 if (a >= k) {
                ans = k;
            } else {
                ans += a;
                k -= a;
                if (b >= k) {
                    k = 0;
                } else {
                    k -= b;
                    ans -= k;
                    }
            } a >= k a k {
                ans = k;
            } ans = k ans k {
                ans += a;
                k -= a;
                if (b >= k) {
                    k = 0;
                } else {
                    k -= b;
                    ans -= k;
                    }
            } ans += a ans a k -= a k a if (b >= k) {
                    k = 0;
                } else {
                    k -= b;
                    ans -= k;
                    } b >= k b k {
                    k = 0;
                } k = 0 k 0 {
                    k -= b;
                    ans -= k;
                    } k -= b k b ans -= k ans k 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 = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());
        int c = Integer.parseInt(sc.next());
        int k = Integer.parseInt(sc.next());
        int ans = 0;

        if (a >= k) {
                ans = k;
            } else {
                ans += a;
                k -= a;
                if (b >= k) {
                    k = 0;
                } else {
                    k -= b;
                    ans -= k;
                    }
            }
        System.out.println(ans);
        }
} public class Main {
    public static void main(String[] args) {

        Scanner sc = new Scanner(System.in);
        int a = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());
        int c = Integer.parseInt(sc.next());
        int k = Integer.parseInt(sc.next());
        int ans = 0;

        if (a >= k) {
                ans = k;
            } else {
                ans += a;
                k -= a;
                if (b >= k) {
                    k = 0;
                } else {
                    k -= b;
                    ans -= k;
                    }
            }
        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 = Integer.parseInt(sc.next());
        int b = Integer.parseInt(sc.next());
        int c = Integer.parseInt(sc.next());
        int k = Integer.parseInt(sc.next());
        int ans = 0;

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

