import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.io.BufferedWriter;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

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

    static class ICoins {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int n = in.nextInt();
            double[] heads = new double[n + 1];
            heads[0] = 1;
            for (int i = 0; i < n; i++) {
                double p = in.nextDouble();
                for (int r = i + 1; r >= 1; r--) {
                    heads[r] = (heads[r] * (1 - p) + heads[r - 1] * p);
                }
                heads[0] = heads[0] * (1 - p);
            }
            double ans = 0;
            for (int i = 0; i <= n; i++) {
                if (i > n - i) ans += heads[i];
            }
            out.println(ans);

        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1 << 16];
        private int curChar;
        private int numChars;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (this.numChars == -1) {
                throw new InputMismatchException();
            } else {
                if (this.curChar >= this.numChars) {
                    this.curChar = 0;

                    try {
                        this.numChars = this.stream.read(this.buf);
                    } catch (IOException var2) {
                        throw new InputMismatchException();
                    }

                    if (this.numChars <= 0) {
                        return -1;
                    }
                }

                return this.buf[this.curChar++];
            }
        }

        public int nextInt() {
            int c;
            for (c = this.read(); isSpaceChar(c); c = this.read()) {
                ;
            }

            byte sgn = 1;
            if (c == 45) {
                sgn = -1;
                c = this.read();
            }

            int res = 0;

            while (c >= 48 && c <= 57) {
                res *= 10;
                res += c - 48;
                c = this.read();
                if (isSpaceChar(c)) {
                    return res * sgn;
                }
            }

            throw new InputMismatchException();
        }

        public static boolean isSpaceChar(int c) {
            return c == 32 || c == 10 || c == 13 || c == 9 || c == -1;
        }

        public double nextDouble() {
            int c;
            for (c = this.read(); isSpaceChar(c); c = this.read()) {
                ;
            }

            byte sgn = 1;
            if (c == 45) {
                sgn = -1;
                c = this.read();
            }

            double res = 0.0D;

            while (true) {
                if (!isSpaceChar(c) && c != 46) {
                    if (c != 101 && c != 69) {
                        if (c >= 48 && c <= 57) {
                            res *= 10.0D;
                            res += (double) (c - 48);
                            c = this.read();
                            continue;
                        }

                        throw new InputMismatchException();
                    }

                    return res * Math.pow(10.0D, (double) this.nextInt());
                }

                if (c == 46) {
                    c = this.read();

                    for (double m = 1.0D; !isSpaceChar(c); c = this.read()) {
                        if (c == 101 || c == 69) {
                            return res * Math.pow(10.0D, (double) this.nextInt());
                        }

                        if (c < 48 || c > 57) {
                            throw new InputMismatchException();
                        }

                        m /= 10.0D;
                        res += (double) (c - 48) * m;
                    }
                }

                return res * (double) sgn;
            }
        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for (int i = 0; i < objects.length; i++) {
                if (i != 0) {
                    writer.print(' ');
                }
                writer.print(objects[i]);
            }
        }

        public void println(Object... objects) {
            print(objects);
            writer.println();
        }

        public void close() {
            writer.close();
        }

    }
}



Measured memory_bytes: 24724