import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.NoSuchElementException;

public class Main {
    static FastScanner scan = new FastScanner();
    static PrintWriter out = new PrintWriter(System.out);

    static int n;
    static int[] a;
    static long k;
    
    public static void main(String[] args) {
        n = scan.nextInt();
        a = new int[n];
        for(int i=0; i<n; i++) {
            a[i] = scan.nextInt();
        }
        k = scan.nextLong();
        
        double l = 0;
        double r = Integer.MAX_VALUE;
        while(r - l > 0.0000000001) {
            double m = (l+r)/2;
            if(comp(m)) {
                l = m;
            } else {
                r = m;
            }
        }
        System.out.printf("%.9f", l);
    }

    static boolean comp(double x) {
        int count = 0;
        for(int i=0; i<a.length; i++) {
            count += a[i] / x;
        }
        return count >= k;
    }
    
    static class FastScanner {
        private final InputStream in = System.in;
        private final byte[] buffer = new byte[1024];
        private int ptr = 0;
        private int buflen = 0;

        private boolean hasNextByte() {
            if (ptr < buflen) {
                return true;
            } else {
                ptr = 0;
                try {
                    buflen = in.read(buffer);
                } catch (IOException e) {
                    e.printStackTrace();
                }
                if (buflen <= 0) {
                    return false;
                }
            }
            return true;
        }

        private int readByte() {
            if (hasNextByte())
                return buffer[ptr++];
            else
                return -1;
        }

        private static boolean isPrintableChar(int c) {
            return 33 <= c && c <= 126;
        }

        private void skipUnprintable() {
            while (hasNextByte() && !isPrintableChar(buffer[ptr]))
                ptr++;
        }

        public boolean hasNext() {
            skipUnprintable();
            return hasNextByte();
        }

        public String next() {
            if (!hasNext())
                throw new NoSuchElementException();
            StringBuilder sb = new StringBuilder();
            int b = readByte();
            while (isPrintableChar(b)) {
                sb.appendCodePoint(b);
                b = readByte();
            }
            return sb.toString();
        }

        public int nextInt() {
            return (int)nextLong();
        }
        
        public long nextLong() {
            if (!hasNext())
                throw new NoSuchElementException();
            long n = 0;
            boolean minus = false;
            int b = readByte();
            if (b == '-') {
                minus = true;
                b = readByte();
            }
            if (b < '0' || '9' < b) {
                throw new NumberFormatException();
            }
            while (true) {
                if ('0' <= b && b <= '9') {
                    n *= 10;
                    n += b - '0';
                } else if (b == -1 || !isPrintableChar(b)) {
                    return minus ? -n : n;
                } else {
                    throw new NumberFormatException();
                }
                b = readByte();
            }
        }
    }
}