#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MOD = 1000000007;

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

ll dp[60];

int main(){
    int n;
    cin >> n;
    for (int i = 0; i <= 2; i++) dp[i] = i;
    for (int i = 3; i <= n; i++) dp[i] = dp[i - 1] + dp[i - 2];

    cout << dp[n] << endl;

    return 0;
}