// 1a
public int FibCalcSum(int fib)
{
    if(fib < 0)
        throw new ArgumentException("Fib number must be greater than or equal to 0");
    if(fib < 2)
        return fib;

    return FibCalcSum(fib - 1) + FibCalcSum(fib - 2);
}

// 1b
// Fuck no loops cause only people that memorized the fib equation would know the answer
// so here's the answer that a normal interviewer would accept

public int FibCalcSum(int fib)
{
    if(fib < 0)
        throw new ArgumentException("Fib number must be greater than or equal to 0");
    if(fib < 2)
        return fib;

    int prevPrevNum = 0;
    int prevNum = 1;
    int currentNum = prevPrevNum + prevNum;

    for(int i = 2; i <= fib; i++)
    {
        currentNum = prevPrevNum + prevNum;
        prevPrevNum = prevNum;
        prevNum = currentNum;
    }

    return currentNum;
}