#include <bits/stdc++.h>

using namespace std;
//用例：
/*
20

输出：
10946
*/
const int N = 110;
int f[N];
int n;
int solve() {
    f[0] = 1, f[1] = 2;
    for (int i = 2; i < n; i++)
        f[i] = f[i - 1] + f[i - 2];
    return f[n - 1];
}
int main() {
    scanf("%d", &n);
    printf("%d\n", solve());
    return 0;
}