[a,b,c] [a,b,c,] [a,b,c] [a,b,c]
dp[N + 150][5];

dp[0][2] = 1;
for(int i = 1; i <= n; i++){
	dp[i][1] += dp[i-1][1] + dp[i-1][2] + dp[i-1][3];
  dp[i][2] += dp[i-1][2] + dp[i-1][3];
  dp[i][3] += dp[i-1][1] + dp[i-1][2] + dp[i-1][3];
}
cout << dp[n][1] + dp[n][2] + dp[n][3];