#include<bits/stdc++.h>
using namespace std;
int main() {
    int a[22] = {0,1,1};
    for(int i = 3;i <= 20;i++) {
        a[i] = a[i - 1] + a[i - 2];
    }
    int b;
    cin>>b;
    cout<<a[b];
}