#pragma comment(linker, "/STACK:65777216")
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <set>
#include <stack>
#include <map>
#include <queue>
#include <string>
#include <memory.h>
#include <iterator>
#define y1 trololoy1
#define y0 trololoy0
#define mem(A,X) memset(A,X,sizeof(A))
#define memo(A) memset(A,0,sizeof(A))
#define forn(I,B) for (int I=1;I<=(B);I++)
#define forg(H,V) for (int H=first[V];h;h=next[H])
#define rep(I,B) for (int I=0;I<(B);I++)
#define labs(X) (((X)>0)?(X):(-(X)))
#define ropen(X) freopen(X,"r",stdin)
#define wopen(X) freopen(X,"w",stdout)
#define rwopen(X) freopen(X".in","r",stdin);freopen(X".out","w",stdout)
#define pb push_back
#define mp make_pair
#define all(X) (X).begin(),(X).end()
#define sqr(X) ((X)*(X))

using namespace std;

typedef pair <int,int> pii;
typedef double ld;
typedef long long ll;
typedef pair <ll,ll> pll;
typedef vector<int> vi;
const int N=111111;
const int INF=111111111;
const double eps=1e-9;
const double pi=3.14159265358979;

int d[1001][1001],n,m,a[N],b[1001];

inline bool check(int k){
    int ans=0;
    forn(i,1000) for (int j=i+1;j<=1000;j++) if (b[i] && b[j] && d[i][j]>=k) ++ans;
    return ans<=m;
}

int main(){
    scanf("%d%d",&n,&m);
    mem(d,63);
    forn(i,n){
        scanf("%d",&a[i]);
        b[a[i]]=1;
    }
    forn(i,n) forn(j,i-1){
        int l=a[i],r=a[j];
        if (l>r) swap(l,r);
        d[l][r]=min(d[l][r],i-j-1);
    }
    forn(k,n-2){
        if (check(k)){
            printf("%d\n",k);
            return 0;
        }
    }
}