class Stat
  {
  public:
    
  Stat() : n(0), m(0.), m2(0.) {}
    
    void Clear()
    {
      n = 0;
      m=0;
      m2=0;
    }
    
        void Push(double x)
        {
	  
	  m+=x;
	  m2+=x*x;
            n++;
        }

        int NumDataValues() const
        {
            return n;
        }

        double Mean() const
        {
            return (n > 0) ? m/n : 0.0;
        }

        double Variance() const
        {
	  return ( (n >0 ) ? (m2/n-m*m/(n*n)) : 0.0 );
        }

        double StandardDeviation() const
        {
            return sqrt( Variance() );
        }

    private:
        int n;
        double m, m2;
    };