tpa lab10 prop O(n)

#include <bits/stdc++.h>
using namespace std;
const string FNAME = "data";
ifstream fin(FNAME + ".in");
ofstream fout(FNAME + ".out");
constexpr int MOD = 1'000'000'007;//'

int main() {
    int n;
    fin >> n;
    int dp[3] = {1, 3};
    for (int i = 3; i <= n; i++) {
        dp[2] = (2LL * dp[0] % MOD + dp[1]) % MOD;
        dp[0] = dp[1];
        dp[1] = dp[2];
    }
    fout << dp[2] << endl;
    fout.close();
    return 0;
}