#include <bits/stdc++.h>
using namespace std;

#define ll long long
#define rep(i,n) for(long long i = 0; i < (long long)(n); i++)

int main(void) {
    ll n; cin >> n; assert(1<=n&&n<=50);
    ll dp[n+10];
    rep(i, n+10) dp[i] = 0;
    dp[0] = 1;
    rep(i, n) {
        dp[i+1] += dp[i];
        dp[i+2] += dp[i];
    }
    cout << dp[n] << endl;
    return 0;
}