Contest1339/D.cpp

#include <bits/stdc++.h>
 
using namespace std;
typedef unsigned long long LL;
const int MAXN = 1000000 + 10;
const LL MOD = 1000000007;
LL f[MAXN][2][2] = {0};
 
inline LL mpower(LL a, int x) {
    a %= MOD;
    if (x == 1 || a == 0)return a;
    if (x == 0)return 1;
    if (x & 1) return (a * mpower(a * a % MOD, x >> 1)) % MOD;
    return mpower(a * a % MOD, x >> 1);
}
 
int main() {
    int n;
    cin >> n;
    if (n < 3) {
        cout << 0 << endl;
        return 0;
    }
 
    f[3][0][0] = f[3][0][1] = f[3][1][0] = 2, f[3][1][1] = 1;
    for (int i = 4; i <= n; i++) {
        f[i][0][0] = (f[i - 1][0][0] + f[i - 1][0][1]) % MOD;
        f[i][0][1] = (f[i - 1][1][1] + f[i - 1][1][0]) % MOD;
        f[i][1][0] = (f[i - 1][0][0] + f[i - 1][0][1]) % MOD;
        f[i][1][1] = f[i - 1][1][0];
    }
    cout << ((mpower(2, n) - (f[n][0][0] + f[n][0][1] + f[n][1][0] + f[n][1][1])) % MOD + MOD) % MOD << endl;
    return 0;
}