Language: C++

#include <algorithm>
#include <iostream>
#include <fstream>
#include <sstream>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <list>
#include <map>
#include <set>
#include <cmath>
#include <limits>
#include <random>
#include <numeric>
#include <iomanip>
#include <cstdlib>
#include <iterator>
#include <functional>
#include <unordered_map>
#include <unordered_set>

using namespace std;

typedef unsigned                    ui;
typedef int                         si;
typedef long                        sl;
typedef long long                   ll;
typedef unsigned long long          ull;

// ----------------

template <typename A, typename B>
string to_string(pair<A, B> p);

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p);

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p);

string to_string(const string& s) {
  return '"' + s + '"';
}

string to_string(const char* s) {
  return to_string((string) s);
}

string to_string(const char ch) {
    string str;
    str.push_back('\'');
    str.push_back(ch);
    str.push_back('\'');
    return str;
}

string to_string(bool b) {
  return (b ? "true" : "false");
}

string to_string(vector<bool> v) {
  bool first = true;
  string res = "{";
  for (int i = 0; i < static_cast<int>(v.size()); i++) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(v[i]);
  }
  res += "}";
  return res;
}

template <size_t N>
string to_string(bitset<N> v) {
  string res = "";
  for (size_t i = 0; i < N; i++) {
    res += static_cast<char>('0' + v[i]);
  }
  return res;
}

template <typename A>
string to_string(A v) {
  bool first = true;
  string res = "{";
  for (const auto &x : v) {
    if (!first) {
      res += ", ";
    }
    first = false;
    res += to_string(x);
  }
  res += "}";
  return res;
}

template <typename A, typename B>
string to_string(pair<A, B> p) {
  return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}

template <typename A, typename B, typename C>
string to_string(tuple<A, B, C> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ")";
}

template <typename A, typename B, typename C, typename D>
string to_string(tuple<A, B, C, D> p) {
  return "(" + to_string(get<0>(p)) + ", " + to_string(get<1>(p)) + ", " + to_string(get<2>(p)) + ", " + to_string(get<3>(p)) + ")";
}

void debug_out() { cerr << endl; }

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
  cerr << " " << to_string(H);
  debug_out(T...);
}

#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

// ----------------

bool Random() {
    std::random_device r;
    std::default_random_engine e1(r());
    std::uniform_int_distribution<int> n_dist(1, 5);

    ll n = n_dist(e1);

    return true;
}

ll func(ll x) {
    if (x < 2) {
        return 1;
    }
    return x * func(x - 2);
}

unordered_map<ll, ll> fact(ll x) {
    unordered_map<ll, ll> factors;

    ll cur = x;
    for (int i = 2; i * i <= x; ++i) {
        while (cur % i == 0) {
            factors[i] += 1;
            cur /= i;
        }
    }

    return factors;
}

int main(int argc, char *argv[])
{
    std::ios_base::sync_with_stdio(false);

    ll n;
    cin >> n;

    ll answer = 0;
    // {
    //     ll o2 = 0;
    //     ll o5 = 0;
    //
    //     for (ll i = 2; i <= n; ++i) {
    //         if (i % 5 == 0 && i % 2 == 0) {
    //             o5 += 1;
    //         }
    //         if (i % 2 == 0) {
    //             o2 += 1;
    //         }
    //     }
    //
    //     debug(n, o2, o5);
    // }

    // debug(func(n));
    //
    // {
    //     unordered_map<ll, ll> counts;
    //
    //     ll o5 = 0;
    //     for (ll i = 2; i <= n; ++i) {
    //         if (i % 2 == 0 && i % 5 == 0) {
    //             unordered_map<ll, ll> f = fact(i);
    //             debug(i, f);
    //
    //             o5 += f[5];
    //
    //             counts[f[5]] += 1;
    //         }
    //     }
    //
    //     debug(o5);
    //     debug(counts);
    // }

    debug(answer);
    answer = -1;

    if (n % 2 == 0) {
        ll a5 = 0;
        for (ll i = 5; i <= n; i *= 5) {
            a5 += n / (i * 2);

            debug(i, a5);
        }

        answer = a5;
    }

    cout << answer << endl;

    //
    // if (n % 2 == 0) {
    //     ll cur = n;
    //     ll power = 1;
    //     while (cur > 0) {
    //         ll digit = cur % 10;
    //         ll value = (digit * power);
    //
    //         answer += (digit * power) / 10;
    //         a5 += (digit * power) / 5 / 2;
    //         a2 += (digit * power) / 2;
    //
    //         debug(digit, power, cur, a5, a2);
    //
    //         power *= 10;
    //         cur /= 10;
    //     }
    // }

    // debug(answer, a2, a5);

    return 0;
}


Compiler output:
a.cc:77:18: error: 'template<long unsigned int N> std::string to_string' redeclared as different kind of entity
   77 | string to_string(bitset<N> v) {
      |                  ^~~~~~
a.cc:62:8: note: previous declaration 'std::string to_string(std::vector<bool>)'
   62 | string to_string(vector<bool> v) {
      |        ^~~~~~~~~
a.cc:77:18: error: 'bitset' was not declared in this scope
   77 | string to_string(bitset<N> v) {
      |                  ^~~~~~
a.cc:22:1: note: 'std::bitset' is defined in header '<bitset>'; this is probably fixable by adding '#include <bitset>'
   21 | #include <unordered_set>
  +++ |+#include <bitset>
   22 | 
a.cc:77:28: error: 'v' was not declared in this scope
   77 | string to_string(bitset<N> v) {
      |                            ^