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#include <iostream>
#include <set>
#include <unordered_set>
#include <map>
#include <unordered_map>
#include <vector>
#include <tuple>

using namespace std;

typedef long long LL;
typedef long double LLD;
typedef unsigned long long ULL;
typedef vector<int> VI;
typedef vector<pair<int, int>> VPII;
typedef pair<int, int> PII;

#define FOR(x, b, e) for(int x = b; x <= (e); ++x)
#define FORD(x, b, e) for(int x = b; x >= (e); --x)
#define REP(x, n) for(int x = 0; x < (n); ++x)
#define VAR(v, n) __typeof(n) v = (n)
#define ALL(c) (c).begin(), (c).end()
#define SIZE(x) ((int)(x).size())
#define FOREACH(i, c) for(VAR(i, (c).begin()); i != (c).end(); ++i)
#define PB push_back
#define ST first
#define ND second
#define INF 1000000001
#define MP make_pair

#ifndef ONLINE_JUDGE
#define debug(x) cerr << #x <<" "; _print(x); cerr << endl;
#else
#define debug(x)
#endif

void _print(LL t) {cerr << t;}
void _print(int t) {cerr << t;}
void _print(string t) {cerr << t;}
void _print(char t) {cerr << t;}
void _print(LLD t) {cerr << t;}
void _print(double t) {cerr << t;}
void _print(ULL t) {cerr << t;}

template <class T, class V> void _print(pair <T, V> p);
template <class T> void _print(vector <T> v);
template <class T> void _print(set <T> v);
template <class T, class V> void _print(map <T, V> v);
template <class T> void _print(multiset <T> v);
template <class T, class V> void _print(pair <T, V> p) {cerr << "{"; _print(p.ST); cerr << ","; _print(p.ND); cerr << "}";}
template <class T> void _print(vector <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
template <class T> void _print(set <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
template <class T> void _print(multiset <T> v) {cerr << "[ "; for (T i : v) {_print(i); cerr << " ";} cerr << "]";}
template <class T, class V> void _print(map <T, V> v) {cerr << "[ "; for (auto i : v) {_print(i); cerr << " ";} cerr << "]";}

int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

	int n, k;
	cin >> n >> k;
	vector<bool> visited(n + 1, false);
	int tmp, act = 0, res{};
	REP(x, n) {
		cin >> tmp;
		if(!visited[tmp]) {
			visited[tmp] = true;
			res += x - act;
			++act;
		}
		if(act == k)
			break;
	}
	if(act < k)
		cout << -1;
	else
		cout << res;

    return 0;
}