Complexity of algorithms

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C++

how can i calculate the time complexity of fibonacci series .....is there possible three notation of complexities best case worst case average case ...how to calculate this complexities of these three cases for a given program...is every program has three complexities..

What I have tried:

int Fibonacci(int n)
{
    if (n <= 1)
        return n;
    else
        return Fibonacci(n - 1) + Fibonacci(n - 2);
}