int num = 15;
int result = -1;

if (num < 2)
    result = num;
else {
    int prev1 = 1;
    int prev2 = 0;
    for (int i=2; i < num; i++)
    {
        int tmp = prev1;
        prev1 = prev1 + prev2;
        prev2 = tmp;
    };
    result = prev1+prev2;
}
result;