#include <iostream>
using namespace std;
int main(){
  int n;cin >> n;
  long long dp[n+1];
  dp[0] = 0;
  dp[1] = 1;
  dp[2] = 2;
  for(int i = 2; n > i; i++){
    dp[i+1] = dp[i]+dp[i-1];
  }
  cout << dp[n] << endl;
}