C++ Standard Deviation

#include<iostream>
#include<cmath>
using namespace std;
int main()
{
    int i, n;

    cout << "Enter Array Number : ";
    cin >> n;
    cout << endl;

    double a[n];
    double b, m, sum = 0, sd = 0;

    cout << "Enter Array Element : ";
    for(i=0;i<n;i++)
    {
        cin >> a[i];
        sum = sum + a[i];
    }

    m = sum/n;

    for(i=0;i<n;i++)
    {
        sd = sd + pow((a[i] - m), 2);
    }

    b = sqrt(sd/n);

    cout << "Standard Deviation :: " << b << endl;

    return 0;
}