#include <bits/stdc++.h>
using namespace std;
int f(int x)
{
    if (x == 1)
        return 0;
    if (x == 2)
        return 1;
    return f(x - 1) + f(x - 2);
}
int main()
{
    int n;
    cin >> n;
    cout << f(n) << endl;
    return 0;
}