Estrela.cpp

#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.cpp"
#else
#define dbg(...) 42
#endif
#define endl '\n'
#define fastio                      \
  ios_base::sync_with_stdio(false); \
  cin.tie(0);                       \
  cout.tie(0);
#define len(__x) (int)__x.size()
using ll = long long;
using ull = unsigned long long;
using ld = long double;
using vll = vector<ll>;
using pll = pair<ll, ll>;
using vll2d = vector<vll>;
using vi = vector<int>;
using vi2d = vector<vi>;
using pii = pair<int, int>;
using vii = vector<pii>;
using vc = vector<char>;
#define all(a) a.begin(), a.end()
#define pb(___x) push_back(___x)
#define mp(___a, ___b) make_pair(___a, ___b)
#define eb(___x) emplace_back(___x)

// vector<string> dir({"LU", "U", "RU", "R", "RD", "D", "LD", "L"});
// int dx[] = {-1, -1, -1, 0, 1, 1, 1, 0};
// int dy[] = {-1, 0, 1, 1, 1, 0, -1, -1};
vector<string> dir({"U", "R", "D", "L"});
int dx[] = {-1, 0, 1, 0};
int dy[] = {0, 1, 0, -1};

const ll oo = 1e18;
ll N;

map<ll, ll> factorization(ll n) {
  map<ll, ll> ans;
  for (ll i = 2; i * i <= n; i++) {
    ll count = 0;
    for (; n % i == 0; count++, n /= i)
      ;
    if (count) ans[i] = count;
  }
  if (n > 1) ans[n]++;
  return ans;
}
int phi(int n) {
  if (n == 1) return 1;

  auto fs = factorization(n);  // a vctor of pair or a map
  auto res = n;

  for (auto [p, k] : fs) {
    res /= p;
    res *= (p - 1);
  }

  return res;
}

bool check(ll i) {
  ll cur = 1;
  set<ll> vis;
  do {
    vis.insert(cur);
    cur = cur + i;
    if (cur > N) cur -= N;
  } while (cur != 1);
  return len(vis) == N;
}

auto solve() {
  vll ans;
  cout << phi(N) / 2 << endl;
}
int32_t main(void) {
#ifndef LOCAL
  fastio;
#endif

  while (cin >> N) {
    solve();
  }
}

// AC, math, number theory, euler phi