Perimeter of squares in a rectangle

• Time : $O(n)$
• Space : $O(1)$

typedef unsigned long long ull;
class SumFct
{
public:
    static ull perimeter(int n) {
        if(n == 0) return 1ull;
        ull res = 0ull, fibo1 = 1, fibo2 = 1;
        for(ull i = 1; i < n; i++) {
            res += fibo1 * 4;
            ull fibo3 = fibo1 + fibo2;
            fibo1 = fibo2;
            fibo2 = fibo3;
        }
        return res + 4 * fibo1 + 4 * fibo2;
    }
};