Why it calculating Wrong Pisano period

    long long fibonacci_fast(long long n) {

    long long a[n]{0};
    a[0]=0;
    a[1]=1;
    for(int i=2;i<=n;i++)
        a[i]=a[i-1]+a[i-2];

    return a[n];
}

and the fuction to calculate pisano period is

long long get_pisano_period(long long n,long long m){
    vector<long long> vec{0,1};

    for(int i=2;i<=n;i++){
        vec.push_back(fibonacci_fast(i)%m);


    long c{2};

    for(int i=2;i<=n;i++){
    if((vec[i]!=0) && (vec[i+1]!=1)){
    c++;
    }}
    return c;
}

In main function i m just outputing the output of get_pisano_period(n,m)