#include <iostream>
#include <algorithm>
#include <cmath>
#include <vector>
#include <complex>
#include <queue>
#include <set>
#include <map>

using namespace std;

#define REP(i,a,b) for(int i=a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)

typedef long long ll;

ll dp[55];

int main() {

  int N; cin >> N;
  dp[0] = 1;
  REP(i, 1, N+1) {
    dp[i-1] += dp[i];
    if(i-2)dp[i-2] += dp[i];
  }

  cout << dp[N] << endl;

  return 0;
}