How do I convert from base n to base 10 without using strings or arrays?

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C

How do I convert from base n to base 10 without using strings or arrays(where n is an input between 2 and 9)? The number being converted is a long long integer type.

The code that I have tried returns the same number (num) that was input into the function.

What I have tried:

int calcBase10(long long num, int base)
{
int base10;
int quotient;
int remainder;

if(base == 10)
{
base10 = num;
return(base10);
}

quotient = num / base;
remainder = num % base;
if(quotient == 0)
{
base10 = remainder;
}
else
{
base10 = (base * calcBase10(quotient, base) + remainder);
}
return(base10);
}