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

题解：DP

/*
*@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=5000+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];
char str;

int maxl[N];
int dp[N][N];
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]);
    }
    sort(a+1,a+n+1);
    int L=1;
    int R=1;
    while(L<=R&&R<=n){
        if(a[R]-a[L]<=5){
            maxl[R]=R-L+1;
            R++;
        }else{
            L++;
        }
    }
    for(int i=1;i<=n;i++){
        dp[1][i]=max(dp[1][i-1],maxl[i]);
    }
    if(dp[1][n] + k >= n){
        cout << n << endl;
        return 0;
    }
    for(int i=2;i<=k;i++){
        for(int j=1;j<=n;j++){
            for(int l=1;l<=maxl[j];l++){
                dp[i][j] = max(dp[i][j-1],max(dp[i][j],dp[i-1][j-l]+l));
                if(dp[i][j]>=n){
                    cout << n << endl;
                    return 0;
                }
            }
        }
    }
    for(int i=1;i<=k;i++){
        ans=max(dp[i][n],ans);
    }
    cout<<ans<<endl;
    //}

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