#include <iostream>

using namespace std;

typedef unsigned long long ull;

ull N;
ull dp[51];

int main() {
    scanf("%llu", &N);

    dp[0] = 0;
    dp[1] = 1;
    dp[2] = 2;
    for (int i = 3; i <= N; i++) {
        dp[i] = dp[i - 1] + dp[i - 2];
    }

    printf("%llu\n", dp[N]);
}