Octal to Binary

#include <iostream>
#include <cmath>

using namespace std;
long long convertOctalToBinary(int);
long long convertOctalToBinary(int octalNumber) {
  int decimalNumber = 0, i = 0;
  long long binaryNumber = 0;
  while(octalNumber != 0) {
    decimalNumber += (octalNumber%10) * pow(8,i);
    ++i;
    octalNumber/=10;
  }
  i = 1;
  while (decimalNumber != 0){
    binaryNumber += (decimalNumber % 2) * i;
    decimalNumber /= 2;
    i *= 10;
  }
  return binaryNumber;
}