import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        B solver = new B();
        solver.solve(1, in, out);
        out.close();
    }

    static class B {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            // 入力
            long a = Long.parseLong(in.next());
            long b = Long.parseLong(in.next());
            long c = Long.parseLong(in.next());
            long k = Long.parseLong(in.next());
            long ans = 0;
            long tmp = k;

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

            // 出力
            out.println(ans);

        }

    }
}
 import java.io.OutputStream; OutputStream java.io import java.io.IOException; IOException java.io import java.io.InputStream; InputStream java.io import java.io.PrintWriter; PrintWriter java.io import java.util.Scanner; Scanner java.util public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        B solver = new B();
        solver.solve(1, in, out);
        out.close();
    }

    static class B {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            // 入力
            long a = Long.parseLong(in.next());
            long b = Long.parseLong(in.next());
            long c = Long.parseLong(in.next());
            long k = Long.parseLong(in.next());
            long ans = 0;
            long tmp = k;

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

            // 出力
            out.println(ans);

        }

    }
} Main public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        B solver = new B();
        solver.solve(1, in, out);
        out.close();
    } main {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        B solver = new B();
        solver.solve(1, in, out);
        out.close();
    } InputStream inputStream = System.in; inputStream System.in System System.in OutputStream outputStream = System.out; outputStream System.out System System.out Scanner in = new Scanner(inputStream); in new Scanner(inputStream) PrintWriter out = new PrintWriter(outputStream); out new PrintWriter(outputStream) B solver = new B(); solver new B() solver.solve(1, in, out) solver.solve solver 1 in out out.close() out.close out String[] args args static class B {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            // 入力
            long a = Long.parseLong(in.next());
            long b = Long.parseLong(in.next());
            long c = Long.parseLong(in.next());
            long k = Long.parseLong(in.next());
            long ans = 0;
            long tmp = k;

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

            // 出力
            out.println(ans);

        }

    } B public void solve(int testNumber, Scanner in, PrintWriter out) {
            // 入力
            long a = Long.parseLong(in.next());
            long b = Long.parseLong(in.next());
            long c = Long.parseLong(in.next());
            long k = Long.parseLong(in.next());
            long ans = 0;
            long tmp = k;

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

            // 出力
            out.println(ans);

        } solve {
            // 入力
            long a = Long.parseLong(in.next());
            long b = Long.parseLong(in.next());
            long c = Long.parseLong(in.next());
            long k = Long.parseLong(in.next());
            long ans = 0;
            long tmp = k;

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

            // 出力
            out.println(ans);

        } long a = Long.parseLong(in.next()); a Long.parseLong(in.next()) Long.parseLong Long in.next() in.next in long b = Long.parseLong(in.next()); b Long.parseLong(in.next()) Long.parseLong Long in.next() in.next in long c = Long.parseLong(in.next()); c Long.parseLong(in.next()) Long.parseLong Long in.next() in.next in long k = Long.parseLong(in.next()); k Long.parseLong(in.next()) Long.parseLong Long in.next() in.next in long ans = 0; ans 0 long tmp = k; tmp k if (a > 0) {
                if (a >= k) ans += k;
                else ans += a;
                tmp -= a;
                a = a - k;
                k = tmp;
            } a > 0 a 0 {
                if (a >= k) ans += k;
                else ans += a;
                tmp -= a;
                a = a - k;
                k = tmp;
            } if (a >= k) ans += k;
                else ans += a; a >= k a k ans += k ans k ans += a ans a tmp -= a tmp a a = a - k a a - k a k k = tmp k tmp if (a <= 0 && b > 0 && k > 0) {
                tmp -= b;
                b = b - k;
                k = tmp;
            } a <= 0 && b > 0 && k > 0 a <= 0 && b > 0 && k > 0 a <= 0 a 0 b > 0 b 0 k > 0 k 0 {
                tmp -= b;
                b = b - k;
                k = tmp;
            } tmp -= b tmp b b = b - k b b - k b k k = tmp k tmp if (a <= 0 && b <= 0 && c > 0 && k > 0) {
                ans -= k;
            } a <= 0 && b <= 0 && c > 0 && k > 0 c > 0 a <= 0 && b <= 0 && c > 0 && k > 0 a <= 0 a 0 b <= 0 b 0 c > 0 c 0 k > 0 k 0 {
                ans -= k;
            } ans -= k ans k out.println(ans) out.println out ans int testNumber testNumber Scanner in in PrintWriter out out public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        B solver = new B();
        solver.solve(1, in, out);
        out.close();
    }

    static class B {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            // 入力
            long a = Long.parseLong(in.next());
            long b = Long.parseLong(in.next());
            long c = Long.parseLong(in.next());
            long k = Long.parseLong(in.next());
            long ans = 0;
            long tmp = k;

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

            // 出力
            out.println(ans);

        }

    }
} public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        B solver = new B();
        solver.solve(1, in, out);
        out.close();
    }

    static class B {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            // 入力
            long a = Long.parseLong(in.next());
            long b = Long.parseLong(in.next());
            long c = Long.parseLong(in.next());
            long k = Long.parseLong(in.next());
            long ans = 0;
            long tmp = k;

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

            // 出力
            out.println(ans);

        }

    }
} Main
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        B solver = new B();
        solver.solve(1, in, out);
        out.close();
    }

    static class B {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            // 入力
            long a = Long.parseLong(in.next());
            long b = Long.parseLong(in.next());
            long c = Long.parseLong(in.next());
            long k = Long.parseLong(in.next());
            long ans = 0;
            long tmp = k;

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

            // 出力
            out.println(ans);

        }

    }
}

