https://codeforces.com/contest/1133/problem/B

题解：对于一个数a[i]直接mod k，然后桶排序思想，暴力枚举 

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=200000+10;
const int M=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,q;
int ans,cnt,flag,temp,sum;
int a[N];
int b[N];
char str;
struct node{};
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    //ios::sync_with_stdio(false);
    //cin.tie(0);
    //cout.tie(0);
    //scanf("%d",&t);
    //while(t--){
    scanf("%d%d",&n,&k);
    for(int i=1;i<=n;i++){
        scanf("%d",&a[i]);
        b[a[i]%k]++;
    }
    for(int i=0;i<=100;i++){
        for(int j=i;j<=100;j++){
            if((i+j)%k==0&&b[i]>0&&b[j]>0){
                if(i==j){
                    ans+=b[i]/2*2;
                    b[i]=b[i]-b[i]/2*2;
                    continue;
                }
                int minl=min(b[i],b[j]);
                b[i]-=minl;
                b[j]-=minl;
                ans+=2*minl;
                //cout<<i<<j<<endl;
            }
        }
    }
    cout<<ans<<endl;
    //}

#ifdef DEBUG
	printf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC);
#endif
    //cout << "Hello world!" << endl;
    return 0;
}