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cses/introductory-problems/trailing-zeroes.cc

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+#include <bits/stdc++.h>  // {{{
+
+// https://codeforces.com/blog/entry/96344
+
+#pragma GCC optimize("O2,unroll-loops")
+#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
+
+using namespace std;
+
+template <typename T>
+constexpr T MIN = std::numeric_limits<T>::min();
+
+template <typename T>
+constexpr T MAX = std::numeric_limits<T>::max();
+
+template <typename T>
+[[nodiscard]] static T sc(auto &&x) {
+  return static_cast<T>(x);
+}
+
+template <typename T>
+[[nodiscard]] static T sz(auto &&x) {
+  return static_cast<T>(x.size());
+}
+
+using ll = long long;
+using ld = long double;
+template <typename T>
+using vec = std::vector<T>;
+template <typename T, size_t N>
+using arr = std::array<T, N>;
+
+#define ff first
+#define ss second
+#define eb emplace_back
+#define pb push_back
+#define all(x) (x).begin(), (x).end()
+#define rall(x) (x).rbegin(), (x).rend()
+//  }}}
+
+void solve() {
+  int n;
+  cin >> n;
+
+  /*
+     every time 10 goes into it, + 1
+
+     1 * 2 * 3 * 4 * 5 <- 1
+
+     6 * 7 * 8 * 9 * 10
+
+     11 12 13 14 15
+     16 17 18 19 20
+*/
+
+  int ans = 0;
+  int b = 5;
+  while (n / b) {
+    ans += n / b;
+    b *= 5;
+  }
+  cout << ans << endl;
+}
+
+int main() {  // {{{
+  cin.tie(nullptr)->sync_with_stdio(false);
+
+  solve();
+
+  return 0;
+}
+// }}}