How do I get the standard deviation from a .txt file?

This is what I have so far, I had a function for the standard deviation but I srapped it as I was overwhelmed with my errors. The formula for standard deviation is: (n 1 – a)² , (n 2 – a)² , (n 3 – a)² , and so forth. This is also my first time using stackoverflow, so any pointers or direction is much appreciated.

#include <iostream>
#include <fstream>
#include <cstdlib>
#include <ctime>
#include <cmath>

using namespace std;

void fileNumber_generator(ofstream& fout, int maximum);
// Generates numbers 0 - 9 in the file chosen.

double average(ifstream& fin, int maximum);
// Creates the average of all the numbers in the file.

int main()
{
    ifstream fin;
    ofstream fout;
    int maximum = 10;
    srand(time(0));

    fileNumber_generator(fout, maximum);
    cout << average(fin, maximum) << endl;


}

void fileNumber_generator(ofstream &fout, int maximum)
{
    int i;
    double number = 0;

    fout.open("numbers.txt");
    if (fout.fail())
    {
        cout << "File could not open.\n";
        exit(1);
    }

    for (i=0; i < maximum; i++)
    {
        fout << number << endl;
        number++;
    }
    fout.close();
}

double average(ifstream& fin, int maximum)
{
    int i;
    double number = 0, total = 0, average = 0;
    fin.open("numbers.txt");
    if (fin.fail( ))
    {
        cout << "File could not open.\n";
        exit(1);
    }
    if (fin)
        fin >> number;
    while (fin)
    {
        total = total + number;
        i++;
        fin >> number;
    }
    if (i > maximum)
    {
        average = total / maximum;
        return(average);
    }
}