Codeforces Round #544 (Div. 3) B. Preparation for International Women's Day(思维)

#include <bits/stdc++.h>
#define maxn 100005
using namespace std;
#define ll long long
int pre[maxn];

int main()
{
  int n, k;
  cin>>n>>k;
  for(int i=1;i<=n;i++){
    int a;
    scanf("%lld", &a);
    pre[a % k]++;
  }
  int ans = 0;
  for(int i=0;i<=k/2;i++){
    if(i == 0){
        ans += pre[i] / 2;
    }
    else if(i == k / 2){
        if(k % 2 == 0) ans += pre[i] / 2;
        else ans += min(pre[i], pre[k - i]);
    }
    else{
        ans += min(pre[i], pre[k - i]);
    }
  }
  cout<<ans * 2<<endl;
  return 0;
}