import std.algorithm, std.container, std.conv, std.math, std.range, std.typecons, std.stdio, std.string;

auto rdsp(){return readln.splitter;}
void pick(R,T)(ref R r,ref T t){t=r.front.to!T;r.popFront;}
void readV(T...)(ref T t){auto r=rdsp;foreach(ref v;t)pick(r,v);}
void readA(T)(size_t n,ref T[]t){t=new T[](n);auto r=rdsp;foreach(ref v;t)pick(r,v);}

void main()
{
  int n; readV(n);
  double[] p; readA(n, p);

  auto dp = new double[][](n+1, n+1);
  dp[0][] = 0; dp[0][0] = 1;
  foreach (i; 1..n+1)
    foreach (j; 0..n+1) {
      dp[i][j] = dp[i-1][j] * (1-p[i-1]);
      if (j > 0) dp[i][j] += dp[i-1][j-1] * p[i-1];
    }

  writefln("%.10f", dp[$-1][n/2+1..$].sum);
}
