Language: C++

/**
 * code generated by JHelper
 * More info: https://github.com/AlexeyDmitriev/JHelper
 * @author RiaD
 */

#include <iostream>
#include <fstream>

#include <iostream>


#include <iterator>


#include <string>
#include <stdexcept>

#ifndef SPCPPL_ASSERT
	#ifdef SPCPPL_DEBUG
		#define SPCPPL_ASSERT(condition) \
		if(!(condition)) { \
			throw std::runtime_error(std::string() + #condition + " in line " + std::to_string(__LINE__) + " in " + __PRETTY_FUNCTION__); \
		}
	#else
		#define SPCPPL_ASSERT(condition)
	#endif
#endif


/**
* Support decrementing and multi-passing, but not declared bidirectional(or even forward) because
* it's reference type is not a reference.
*
* It doesn't return reference because
* 1. Anyway it'll not satisfy requirement [forward.iterators]/6
*   If a and b are both dereferenceable, then a == b if and only if *a and
*   b are bound to the same object.
* 2. It'll not work with reverse_iterator that returns operator * of temporary which is temporary for this iterator
*
* Note, reverse_iterator is not guaranteed to work  now too since it works only with bidirectional iterators,
* but it's seems to work at least on my implementation.
*
* It's not really useful anywhere except iterating anyway.
*/
template <typename T>
class IntegerIterator: public std::iterator<std::input_iterator_tag, T, std::ptrdiff_t, T*, T> {
public:
	explicit IntegerIterator(T value): value(value) {

	}

	IntegerIterator& operator++() {
		++value;
		return *this;
	}

	IntegerIterator operator++(int) {
		IntegerIterator copy = *this;
		++value;
		return copy;
	}

	IntegerIterator& operator--() {
		--value;
		return *this;
	}

	IntegerIterator operator--(int) {
		IntegerIterator copy = *this;
		--value;
		return copy;
	}

	T operator*() const {
		return value;
	}

	bool operator==(IntegerIterator rhs) const {
		return value == rhs.value;
	}

	bool operator!=(IntegerIterator rhs) const {
		return !(*this == rhs);
	}

private:
	T value;
};

template <typename T>
class IntegerRange {
public:
	IntegerRange(T begin, T end): begin_(begin), end_(end) {
		SPCPPL_ASSERT(begin <= end);
	}

	IntegerIterator<T> begin() const {
		return IntegerIterator<T>(begin_);
	}

	IntegerIterator<T> end() const {
		return IntegerIterator<T>(end_);
	}

private:
	T begin_;
	T end_;
};

template <typename T>
class ReversedIntegerRange {
	typedef std::reverse_iterator<IntegerIterator<T>> IteratorType;
public:
	ReversedIntegerRange(T begin, T end): begin_(begin), end_(end) {
		SPCPPL_ASSERT(begin >= end);
	}

	IteratorType begin() const {
		return IteratorType(IntegerIterator<T>(begin_));
	}

	IteratorType end() const {
		return IteratorType(IntegerIterator<T>(end_));
	}

private:
	T begin_;
	T end_;
};

template <typename T>
IntegerRange<T> range(T to) {
	return IntegerRange<T>(0, to);
}

template <typename T>
IntegerRange<T> range(T from, T to) {
	return IntegerRange<T>(from, to);
}

template <typename T>
IntegerRange<T> inclusiveRange(T to) {
	return IntegerRange<T>(0, to + 1);
}

template <typename T>
IntegerRange<T> inclusiveRange(T from, T to) {
	return IntegerRange<T>(from, to + 1);
}

template <typename T>
ReversedIntegerRange<T> downrange(T from) {
	return ReversedIntegerRange<T>(from, 0);
}

template <typename T>
ReversedIntegerRange<T> downrange(T from, T to) {
	return ReversedIntegerRange<T>(from, to);
}

template <typename T>
ReversedIntegerRange<T> inclusiveDownrange(T from) {
	return ReversedIntegerRange<T>(from + 1, 0);
}

template <typename T>
ReversedIntegerRange<T> inclusiveDownrange(T from, T to) {
	return ReversedIntegerRange<T>(from + 1, to);
}



#include <algorithm>
#include <vector>


template <typename R>
void sort(R& range) {
	std::sort(range.begin(), range.end());
}

template <typename R, typename Comp>
void sort(R& range, Comp comp) {
	std::sort(range.begin(), range.end(), comp);
}

template <typename R>
void reverse(R& range) {
	std::reverse(range.begin(), range.end());
}

template <typename R, typename T>
auto lower_bound(const R& range, const T& value) -> decltype(range.begin()) {
	return std::lower_bound(range.begin(), range.end(), value);
}

template <typename R, typename T, typename Comp>
auto lower_bound(const R& range, const T& value, Comp comp) -> decltype(range.begin()) {
	return std::lower_bound(range.begin(), range.end(), value, comp);
}

template <typename R, typename T>
auto upper_bound(const R& range, const T& value) -> decltype(range.begin()) {
	return std::upper_bound(range.begin(), range.end(), value);
}

template <typename R, typename T, typename Comp>
auto upper_bound(const R& range, const T& value, Comp comp) -> decltype(range.begin()) {
	return std::upper_bound(range.begin(), range.end(), value, comp);
}

template <typename R>
auto min_element(const R& range) -> decltype(range.begin()) {
	return std::min_element(range.begin(), range.end());
}

template <typename R>
auto max_element(const R& range) -> decltype(range.begin()) {
	return std::max_element(range.begin(), range.end());
}

template <typename R>
bool next_permutation(R& range) {
	return std::next_permutation(range.begin(), range.end());
}

template <typename T>
void unique(std::vector<T>& range) {
	range.erase(std::unique(range.begin(), range.end()), range.end());
}

template <typename R>
R sorted(R range) {
	sort(range);
	return range;
}

template <typename R, typename Comp>
R sorted(R range, Comp comp) {
	sort(range, comp);
	return range;
}

template <typename R>
R reversed(R range) {
	reverse(range);
	return range;
}



using namespace std;

class TaskD {
public:
	void solve(std::istream& in, std::ostream& out) {
		int n, m;
		in >> n >> m;
		vector<vector<int>> v(n);
		for (int i: range(n)) {
			int x;
			in >> x;
			v[x % m].push_back(x);
		}

		int64_t ans = 0;
		ans += v[0].size() / 2;
		if (m % 2 == 0) {
			ans += v[m / 2].size() / 2;
		}
		for (int i = 1; i * 2 < m; ++i) {
			ans += min(v[i].size(), v[m - i].size());
			if (v[i].size() > v[m - i].size()) {
				ans += cnt(v[i], v[i].size() - v[m - i].size());
			} else {
				ans += cnt(v[m - i], v[m - i].size() - v[i].size());
			}
		}

		out << ans << "\n";
	}

	int64_t cnt(vector<int>& v, int x) {
		sort(v);
		int64_t ans = 0;
		for (int i = 0; i + 1 < (int)v.size();) {
			if (v[i] == v[i + 1]) {
				i = i + 2;
				++ans;
			} else {
				i = i + 1;
			}
		}
		//cerr << x << ' ' << ans << endl;
		return min(x / 2LL, ans);
	}
};


int main() {
	std::ios_base::sync_with_stdio(false);
	TaskD solver;
	std::istream& in(std::cin);
	std::ostream& out(std::cout);
	in.tie(nullptr);
	out << std::fixed;
	out.precision(20);
	solver.solve(in, out);
	return 0;
}


Compiler output:
a.cc:47:36: warning: 'template<class _Category, class _Tp, class _Distance, class _Pointer, class _Reference> struct std::iterator' is deprecated [-Wdeprecated-declarations]
   47 | class IntegerIterator: public std::iterator<std::input_iterator_tag, T, std::ptrdiff_t, T*, T> {
      |                                    ^~~~~~~~
In file included from /usr/include/c++/14/bits/stl_iterator_base_funcs.h:66,
                 from /usr/include/c++/14/string:47,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:7:
/usr/include/c++/14/bits/stl_iterator_base_types.h:127:34: note: declared here
  127 |     struct _GLIBCXX17_DEPRECATED iterator
      |                                  ^~~~~~~~
a.cc: In member function 'int64_t TaskD::cnt(std::vector<int>&, int)':
a.cc:296:27: error: no matching function for call to 'min(long long int, int64_t&)'
  296 |                 return min(x / 2LL, ans);
      |                        ~~~^~~~~~~~~~~~~~
In file included from /usr/include/c++/14/string:51:
/usr/include/c++/14/bits/stl_algobase.h:233:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::min(const _Tp&, const _Tp&)'
  233 |     min(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:233:5: note:   template argument deduction/substitution failed:
a.cc:296:27: note:   deduced conflicting types for parameter 'const _Tp' ('long long int' and 'int64_t' {aka 'long int'})
  296 |                 return min(x / 2LL, ans);
      |                        ~~~^~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::min(const _Tp&, const _Tp&, _Compare)'
  281 |     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61,
                 from a.cc:174:
/usr/include/c++/14/bits/stl_algo.h:5686:5: note: candidate: 'template<class _Tp> constexpr _Tp std::min(initializer_list<_Tp>)'
 5686 |     min(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5686:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5696:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::min(initializer_list<_Tp>, _Compare)'
 5696 |     min(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5696:5: note:   template argument deduction/substitution failed:
a.cc:296:27: note:   mismatched types 'std::initializer_list<_Tp>' and 'long long int'
  296 |                 return min(x / 2LL, ans);
      |                        ~~~^~~~~~~~~~~~~~