intN;   



	    cin >> N;   



	 



	    vector<double> p(N);



	    for(inti = 0; i < N; i++)



	    {



	        cin >> p[i];



	    }



	    vector<double> p100(100);



	    for(inti = 0; i < 100; i++)



	    {



	        intindex = i % N;



	        p100[i] = p[index];



	    }



	 



	    doublesum = 0.0;



	    doublepre = 1.0;



	    for(inti = 0; i < 50; i+=2)



	    {



	        sum += pre * p100[i];



	        pre *= ((1.0 - p100[i]) * (1.0 - p100[i + 1]));



	    }



	 



	    printf("%.4f\n", sum);