octal to decimal

#include <iostream>
#include<cmath>
#include<ctime>
#include<string>
#include <iomanip>
#include <fstream>

using namespace std;

int octal2Dec(string& octal);
bool validity(int number);
int stringToNum(string number);


int main()
{
	string number;
	getline (cin , number);

	
	cout << octal2Dec(number);
	return 0;
}

int octal2Dec(string& octal)
{
	int digits, decimal = 0, count = 0,oct = stringToNum(octal);
	while (oct != 0)
	{
	
		digits = oct % 10;
		if(validity(digits) == true)
		  decimal += digits* pow(8,count);
		else
		{
			return NAN;
		}
		oct /= 10;
		count++;
		
	}
	return decimal;
}
bool validity(int number)
{
	return number % 8 == 0 || number % 9 == 0? false : true;
		
}
int stringToNum(string number)
{
	int convertedNumber = 0;
	for (int i = 0; i < number.length(); i++)
	{
		convertedNumber = (convertedNumber * 10) + static_cast<int>(number[i] - '0');
	}
	return convertedNumber;
}