E. Tetrahedron

#include <bits/stdc++.h>

using namespace std;
using lli = long long;
const lli N = 1e9 + 7;
lli A[2], B[2], C[2], D[2];

int main(){

    int n;
    scanf("%d", &n);

    D[1] = 1;
    A[1] = B[1] = C[1] = 0;

    for(int i=2;i<=n+1;i++){
        int cur = i % 2;
        int pre = (i - 1) % 2;
        A[cur] = (B[pre] + C[pre] + D[pre]) % N;
        B[cur] = (A[pre] + C[pre] + D[pre]) % N;
        C[cur] = (A[pre] + B[pre] + D[pre]) % N;
        D[cur] = (A[pre] + B[pre] + C[pre]) % N;
        A[pre] = B[pre] = C[pre] = D[pre] = 0;
    }

    printf("%lld", D[(n + 1) % 2]);

    return 0;
}