import strutils, sequtils, algorithm, math

var
  n = readLine(stdin).parseInt()
  dp = newSeq[int](52)

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

echo(dp[n])