#include <iostream>
using ull = unsigned long long;
int main() {
     ull n;
    std::cin >> n;
    
    ull dp[n];;
    dp[1] = 1;
    dp[2] = 2;
    dp[3] = 3;
    
    for (ull i = 4; i <= n; i++) {
        dp[i] = dp[i-1] + dp[i-2];
    }
    std::cout << dp[n] << std::endl;
}