text: w, h = 2, 10003
  dp = [[0 for x in range(w)] for y in range(h)]
  
  n = int(input())
  
  dp[0][0] = dp[0][1] = 1
  
  for i in range(1, n):
  	dp[i][0] += dp[i - 1][1]
  	dp[i][1] += dp[i - 1][1] + dp[i - 1][0]
  
  print(dp[n - 1][1] + dp[n - 1][0])