Fibonnaci numbers

#include "bits/stdc++.h"
using namespace std;
typedef long long int ll;
unordered_map < ll, ll >  f;
const ll mod = 1e9 + 7;
unordered_map < ll, ll > :: iterator it;
ll fib(ll n){

    if(n == 0)return 0;
    if(f[n])return f[n];

    ll k = (n+1)/2;

    if(n%2 == 0){
        return f[n] = ((2 * fib(k-1) % mod + fib(k) % mod) % mod * fib(k) % mod) % mod;
    }
    else{
        return f[n] = (fib(k) * fib(k) % mod + fib(k-1) * fib(k-1) % mod) % mod;
    }

}
int main(){
 ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
   ll n, gcd, num, i;
    f[0] = 0;
    f[1] = 1;
    f[2] = 1;
    cin >> n;
   cout << fib(n);
    return 0;
}