Language: C++

#include <bits/stdc++.h>
using namespace std;

//type
#define ll long long
typedef pair<int, int> P;
//定数
#define INF 1000000000000 //10^12:∞
#define MOD 1000000007    //10^9+7:合同式の法
#define MAXR 100000       //10^5:配列の最大のrange
//略記
#define PB push_back //挿入
#define MP make_pair //pairのコンストラクタ
#define F first      //pairの一つ目の要素
#define S second     //pairの二つ目の要素
#define Z class

// input
#define INIT                          \
    std::ios::sync_with_stdio(false); \
    std::cin.tie(0);
#define VAR(type, ...) \
    type __VA_ARGS__;  \
    MACRO_VAR_Scan(__VA_ARGS__);
template <typename T>
void MACRO_VAR_Scan(T &t)
{
    std::cin >> t;
}
template <typename First, typename... Rest>
void MACRO_VAR_Scan(First &first, Rest &... rest)
{
    std::cin >> first;
    MACRO_VAR_Scan(rest...);
}
#define VEC_ROW(type, n, ...)                \
    std::vector<type> __VA_ARGS__;           \
    MACRO_VEC_ROW_Init(n, __VA_ARGS__);      \
    for (int w_ = 0; w_ < n; ++w_)           \
    {                                        \
        MACRO_VEC_ROW_Scan(w_, __VA_ARGS__); \
    }
template <typename T>
void MACRO_VEC_ROW_Init(int n, T &t)
{
    t.resize(n);
}
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Init(int n, First &first, Rest &... rest)
{
    first.resize(n);
    MACRO_VEC_ROW_Init(n, rest...);
}
template <typename T>
void MACRO_VEC_ROW_Scan(int p, T &t) { std::cin >> t[p]; }
template <typename First, typename... Rest>
void MACRO_VEC_ROW_Scan(int p, First &first, Rest &... rest)
{
    std::cin >> first[p];
    MACRO_VEC_ROW_Scan(p, rest...);
}
#define VEC(type, c, n)     \
    std::vector<type> c(n); \
    for (auto &i : c)       \
        std::cin >> i;
// 2d vector:  MAT(type, v, size, size);
#define MAT(type, c, m, n)                                     \
    std::vector<std::vector<type>> c(m, std::vector<type>(n)); \
    for (auto &R : c)                                          \
        for (auto &w : R)                                      \
            std::cin >> w;

// output
template <typename T>
void MACRO_OUT(const T t) { std::cout << t; }

#define OUT(...) MACRO_OUT(__VA_ARGS__);
#define SP std::cout << " ";
#define TAB std::cout << "\t";
#define BR std::cout << "\n";
#define ENDL std::cout << std::endl;

#define YES() printf("YES\n")
#define NO() printf("NO\n")
#define isYES(x) printf("%s\n", (x) ? "YES" : "NO")
#define Yes() printf("Yes\n")
#define No() printf("No\n")
#define yes() printf("yes\n")
#define no() printf("no\n")
#define ln cout << '\n'
template <Z A>
void pr(A a)
{
    cout << a;
    ln;
}
template <Z A, Z B>
void pr(A a, B b)
{
    cout << a << ' ' << b << endl;
}

// OTHER
// xの二乗を返す (関数テンプレート版)
template <typename T>
T square(T x) { return x * x; }
template <class T>
T GCD(T a, T b)
{
    T r;
    while (a)
        r = b, b = a, a = r % a;
    return b;
}
template <class T>
T LCM(T a, T b) { return a / GCD(a, b) * b; }

#define chmax(x, y) (x = max(x, y))
#define chmin(x, y) (x = min(x, y))
// loop
#define rep(i, n) for (ll i = 0; i < (n); i++)
#define repe(i, n) for (ll i = 0; i <= (n); i++)
#define repd(i, n) for (ll i = n; i > 0; i--)
#define repde(i, n) for (ll i = n; i >= 0; i--)
#define FOR(i, a, b) for (ll i = a; i < (b); i++)
#define FORE(i, a, b) for (ll i = a; i <= (b); i++)
#define FORD(i, a, b) for (ll i = a; i >= (b); i--)
#define FORA(i, I) for (const auto &i : I)

// vector
#define ALL(x) x.begin(), x.end()
typedef vector<int> vi;
typedef vector<string> vs;
typedef vector<vi> vvi;
typedef vector<ll> vl;
#define ERASE(x) x.erase(unique(ALL(x)), x.end()); // erase same elements

bool yes = 0;
bool boo = 0;
int maxx = -100000, minn = 100000;
int sum = 0;
int ans = 0;
int count = 0;

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
// cout << (conditon ? (result:  == 1 ): (==0)) << endl; <- output result of if
// long long 型が表せる値の上限はおよそ 9 × 10＾18 です．

// vector
// sum: accumulate(ALl(x),0)
signed main()
{
    // abc 136 B

    VAR(int, x);
    rep(i, x)
    {
        // 桁数を数える
        while (x > 0)
        {
            x /= 10;
            count++;
        }
        if (count % 2 == 1)
            ans++;
    }
    pr(ans);

    return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:163:13: error: reference to 'count' is ambiguous
  163 |             count++;
      |             ^~~~~
In file included from /usr/include/c++/14/algorithm:86,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/pstl/glue_algorithm_defs.h:101:1: note: candidates are: 'template<class _ExecutionPolicy, class _ForwardIterator, class _Tp> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, typename std::iterator_traits<_II>::difference_type> std::count(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, const _Tp&)'
  101 | count(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value);
      | ^~~~~
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:4025:5: note:                 'template<class _IIter, class _Tp> typename std::iterator_traits< <template-parameter-1-1> >::difference_type std::count(_IIter, _IIter, const _Tp&)'
 4025 |     count(_InputIterator __first, _InputIterator __last, const _Tp& __value)
      |     ^~~~~
a.cc:143:5: note:                 'int count'
  143 | int count = 0;
      |     ^~~~~
a.cc:165:13: error: reference to 'count' is ambiguous
  165 |         if (count % 2 == 1)
      |             ^~~~~
/usr/include/c++/14/pstl/glue_algorithm_defs.h:101:1: note: candidates are: 'template<class _ExecutionPolicy, class _ForwardIterator, class _Tp> __pstl::__internal::__enable_if_execution_policy<_ExecutionPolicy, typename std::iterator_traits<_II>::difference_type> std::count(_ExecutionPolicy&&, _ForwardIterator, _ForwardIterator, const _Tp&)'
  101 | count(_ExecutionPolicy&& __exec, _ForwardIterator __first, _ForwardIterator __last, const _Tp& __value);
      | ^~~~~
/usr/include/c++/14/bits/stl_algo.h:4025:5: note:                 'template<class _IIter, class _Tp> typename std::iterator_traits< <template-parameter-1-1> >::difference_type std::count(_IIter, _IIter, const _Tp&)'
 4025 |     count(_InputIterator __first, _InputIterator __last, const _Tp& __value)
      |     ^~~~~
a.cc:143:5: note:                 'int count'
  143 | int count = 0;
      |     ^~~~~