问map似乎找到了不存在的元素

EN

#include <bits/stdc++.h>

using namespace std;
std::map<unsigned long long,unsigned long long> F;
unsigned long long fib(unsigned long long n)
{
    if(n==-1) return 0; // Shifting index by 1
    if(n==0) return 1;
    if(n==1) return 1;
    if(F.find(n) != F.end()) return F[n]; // This seems to be the problem,
    else
    {
        if(n%2==0) //
        {
            F[n/2-1] = fib(n/2-1)%1000000007;
            F[n/2] = fib(n/2)%1000000007;
            return F[n] = (F[n/2-1]*F[n/2-1]+F[n/2]*F[n/2])%1000000007;
        }
        else
        {
            F[n/2] = fib(n/2)%1000000007;
            F[n/2+1] = fib(n/2+1)%1000000007;
            return F[n] = (F[n/2]*(2*F[n/2+1]-F[n/2]))%1000000007;
        }
    }
}
int main() {
    unsigned long long int broj; 
    cin >> broj; // input the number
    cout << fib(broj-1) << endl;
    return 0;
}