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#include <stdio.h>
#include <time.h>
#include <math.h>
#include <stdlib.h>
#include <string.h>
#include <map> 
#include <string>
#include <vector>  
#include <iostream> 
#include <sstream> 
#include <queue>
#include <algorithm>
#include <assert.h>


using namespace std;

#define ll long long
#define PB 		push_back
#define FOR(a,start,end) 	for(int a=int(start); a<int(end); a++)
#define INF 		INT_MAX
#define SORT(a) 	sort(a.begin(),a.end()) 
#define CL(a,x) 		memset(a,x,sizeof(a))
#define REP(a,x)	for(int a=0;a<x;a++)
#define REP1(a,x)	for(int a=1;a<=x;a++)
#define MP 		make_pair



typedef vector<ll>     vi;
typedef pair<ll, ll>     pii;
typedef vector<string> vs;
typedef vector<pii> vii;
typedef vector<vector<ll> > vvi;
typedef vector<vector<string> > vvs;
typedef vector<pair<string, string> > vss;
typedef pair<string, string> pss;
typedef pair<ll, pii> ppii;
typedef vector<ppii> vppii;
typedef vector<vector<pii> > vvii;
typedef vector<vvi> vvvi;


ll n, k, a, b, c, z, m, d, p;

vi vk, va, vb,vc;
string w;

int main() {
//	freopen("c:\\wojtek\\uva\\pa\\debug\\t1.in", "rt", stdin);
//	while (1) {
		cin >> n >> k;
		vk.assign(n + 1, 0);
		va.assign(n, 0);
		vb.assign(n + 1, 0);
		vc.assign(n + 1, 0);
		z = 0;
		c = 0;
		for (int i = 0; i < n; i++) {
			cin >> va[i];
		}
		for (int i = 0; i < k; i++) {
			vb[va[i]]++;
			if (vb[va[i]] == 2)
				vk[va[i]] = 1;
			else if (vb[va[i]] == 1)
				c++;
		}
		for (int i = k; i < n; i++) {
			if (vc[va[i]] == 0) {
				vc[va[i]]++;
				if (vb[va[i]] == 0)
					c++;
			}
		}
		if (c < k)
			z = -1;
		else {
			d = k - 1;
			for (int i = k - 1; i >= 0; i--) {
				if (vb[va[i]] > 1) {
					z += d - i;
					d--;
					vb[va[i]]--;
				}
				else
					vb[va[i]] *= -1;
			}
			d++;
			if (d < k) {
				for (int i = k; i < n; i++) {
					if (vb[va[i]] == 0) {
						z += i - d;
						d++;
						if (d == k)
							break;
						vb[va[i]] = -1;
					}
				}
			}
		}

		cout << z << endl;
//	}
	return 0;

}