int N;     cin >> N;     vector<double> p(N);     for (int i = 0; i < N; i++)     {         cin >> p[i];     }     vector<double> p100(100);     for (int i = 0; i < 100; i++)     {         int index = i % N;         p100[i] = p[index];     }     double sum = 0.0;     double pre = 1.0;     for (int i = 0; i < 50; i+=2)     {         sum += pre * p100[i];         pre *= ((1.0 - p100[i]) * (1.0 - p100[i + 1]));     }     printf("%.4f\n", sum);