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//#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
//~ while (clock()<=69*CLOCKS_PER_SEC)

#define ll long long
#define ld long double
#define pi pair<int,int>
#define pd pair<ld,ld>
#define ft first
#define sd second
#define st first
#define nd second
#define mp make_pair
#define pb push_back
#define eb emplace_back
#define FOR(i,a,b) for(int i=(a); i<=(b);i++)
#define F(i,a,b) FOR(i,(a),(b)-1)
#define REV(i,a,b) for(int i=(a); i>=(b);i--)
#define VI vector<int>
#define VPI vector<pi>
#define VPD vector<pd>
#define PI 3.14159265
#define all(x) (x).begin(), (x).end()
#define sz(a) (int)((a).size())
#define int long long

template<class TH> void _dbg(const char *sdbg, TH h){cerr<<sdbg<<"="<<h<<"\n";}
template<class TH, class... TA> void _dbg(const char *sdbg, TH h, TA... a) {
	while(*sdbg!=',')cerr<<*sdbg++;cerr<<"="<<h<<","; _dbg(sdbg+1, a...);
}
#define debug(...) _dbg(#__VA_ARGS__, __VA_ARGS__)
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
const long long K = 1000;
const long long KK = K*K;
const long long MIL = KK*K;
const long long INF = MIL*MIL;
const long long MOD = 998244353;
const long long N = 1e5 + 10, M=10;

void solve() {
	int n, k;
	cin >> n >> k;
	VI v(n);

	F(i, 0, n) {
		cin >> v[i];
	}

	int res = 0;
	vector<bool> bylo(n + 1);
	int ktore = 0;
	F (i, 0, n)  {
		if (!bylo[ v[i] ])	 {
			bylo[ v[i] ] = true;
			res += i - ktore;
			ktore++;
		}

		if (ktore == k) {
			break;
		}
	}

	if (ktore != k) {
		cout << "-1\n";
		return;
	}

	cout << res << "\n";
}

int32_t main()
{
	//	freopen("input.txt", "r", stdin);
	//	freopen("output.txt", "w", stdout);

	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	cerr.tie(0);
	cout<<setprecision(9)<<fixed;
	cerr<<setprecision(6)<<fixed;
	int test = 1, f;
	//    cin>>test;
	F(_test, 0, test){
		//cout<<"Case #"<<_test + 1<<": ";
		solve();
		//		if(_test == 1)
		//            return 0;
	}
}
/*

 */