How to find fibonacci sums of huge numbers?

I'm solving a CSES problem in which I've to find the sum of first 'n' Fibonacci numbers. The code:

#pragma GCC optimize("Ofast")
#include <iostream>
 
using namespace std;
 
int main()
{
    unsigned long long int n;
    scanf("%llu", &n);
    unsigned long long int seq[n];
    seq[0] = 0;
    seq[1] = 1;
    unsigned long long int mod = 1000000000 + 7;
    for (unsigned long long int i = 2; i < n + 1; i++) {
        seq[i] = (seq[i - 1] + seq[i - 2]) % mod;
    }
    cout << seq[n];
}

The problem specifies that the value of n can get upto 10^18 and therefore I have used unsigned long long int to initialize n. The problem also instructs to give the modulo 7 answer. The code is working fine for values of n upto 4 digits but breaks when the value of n rises to the upper ceiling of 10^18.It gives a (0xC00000FD) error and does not return anything. Please help me understand the problem here and how to deal with it. Any other suggestions would also be appreciated.