1133B - Preparation for International Women's Day - CodeForces Solution

#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define nl cout << "\n"
#define in_sort(v) sort(v.begin(),v.end())
#define de_sort(d) sort(d.begin(),d.end(),greater<ll>())
#define loop(x,y) for(ll i = x;i < y;i++)
#define ts   ll t; cin >> t;while
#define speedUp ios_base::sync_with_stdio(0), cin.tie(0),cout.tie(0);
using namespace std;
ll gcd(ll a, ll b) { if (a == 0) return b;  return gcd(b % a, a); }
ll lcm(ll a, ll b) { return a / gcd(a,b) * b; }
int fastpower(int x, int power) { int result = 1; while (power > 0) { if (power % 2 == 1)result = result * x; x = x * x; power /= 2; } return result; }
ll mod_fastpower(ll x, ll power, ll m) { ll result = 1; while (power > 0) { if (power % 2 == 1)result = (result % m * x % m) % m; x = ((x % m) * (x % m)) % m; power /= 2; } return result; }
using namespace std;
ll v[106];
int main()
{
    speedUp;
    //freopen("input.txt", "r", stdin);
    //freopen("output.txt", "w", stdout);
    ll n, k , ans = 0; cin >> n >> k;
    map <ll, ll> m;
    loop(0, n) {
        ll x; cin >> x;
        v[x % k]++;
    }
    for (ll i = 1; i < (k+1)/2; i++) {
        ans += min(v[i], v[k - i]);
    }
    if (k % 2 == 0) ans += v[k / 2] / 2;
    ans += v[0] / 2;
    cout << 2 * ans;
    



}