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

/**
 * 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;
        FastScanner in = new FastScanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskI solver = new TaskI();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskI {
        public void solve(int testNumber, FastScanner in, PrintWriter out) {
            int n = in.nextInt();

            double[] dp = new double[(n + 1) * (n + 2) / 2];
            dp[0] = 1;
            for (int i = 1; i <= n; i++) {
                double p = in.nextDouble();
                int ci = i * (i + 1) / 2;
                int cp = i * (i - 1) / 2;
                for (int j = 0; j <= i; j++) {
                    if (j > 0)
                        dp[ci + j] += dp[cp + j - 1] * p;
                    if (j < i)
                        dp[ci + j] += dp[cp + j] * (1 - p);
                }
            }

            double ans = 0;
            for (int i = dp.length - (n + 1) / 2; i < dp.length; i++)
                ans += dp[i];

            out.println(ans);
        }

    }

    static class FastScanner {
        private InputStream in;
        private byte[] buffer = new byte[1024];
        private int bufPointer;
        private int bufLength;

        public FastScanner(InputStream in) {
            this.in = in;
        }

        private int readByte() {
            if (bufPointer >= bufLength) {
                if (bufLength == -1)
                    throw new InputMismatchException();

                bufPointer = 0;
                try {
                    bufLength = in.read(buffer);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }

                if (bufLength <= 0)
                    return -1;
            }
            return buffer[bufPointer++];
        }

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

        private static boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public String next() {
            StringBuilder sb = new StringBuilder();

            int b = readByte();
            while (!isPrintableChar(b))
                b = readByte();

            while (isPrintableChar(b)) {
                sb.appendCodePoint(b);
                b = readByte();
            }

            return sb.toString();
        }

        public long nextLong() {
            long n = 0;

            int b = readByte();
            while (isSpaceChar(b))
                b = readByte();

            boolean minus = (b == '-');
            if (minus)
                b = readByte();

            while (b >= '0' && b <= '9') {
                n *= 10;
                n += b - '0';
                b = readByte();
            }

            if (!isSpaceChar(b))
                throw new NumberFormatException();

            return minus ? -n : n;
        }

        public int nextInt() {
            long n = nextLong();

            if (n < Integer.MIN_VALUE || n > Integer.MAX_VALUE)
                throw new NumberFormatException();

            return (int) n;
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

    }
}



Measured memory_bytes: 59860