#include <iostream>
using namespace std;

int main() {
  int n;
  uint64_t dp[51];
  dp[0] = 0;
  dp[1] = 1;
  cin >> n;

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

  cout << dp[n] << endl;

  return 0;
}