#include <iostream>
using namespace std;
int main(void){
    unsigned long long list[51] = {0, 1, 2};
    
    int n;
    cin >> n;
    
    for(int i = 3; i <= n; i++)
        list[i] = list[i - 2] + list[i - 1];
    
    cout << list[n] << '\n';
}