Without 3

n = int(input())
dp = []
for i in range(n + 1):
  dp.append([0] * 3)

dp[0][0] = 1

for i in range(1, n+1):
  dp[i][0] = dp[i-1][0] + dp[i-1][1] + dp[i-1][2]
  dp[i][1] = dp[i-1][0]
  dp[i][2] = dp[i-1][1]

print(dp[n][0] + dp[n][1] + dp[n][2])