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// Problem: Oranzada [B]
// https://sio2.mimuw.edu.pl/c/pa-2021-1/p/ora/

#include <cstdio>
#include <cmath>
#include <iostream>
#include <algorithm>
#include <string>
#include <cctype>
#include <vector>
#include <set>
#include <map>
using namespace std;
#define REP(i,n) for(int _n=(n), i=0;i<_n;++i)
#define FOR(i,a,b) for(int i=(a),_b=(b);i<=_b;++i)
#define FORD(i,a,b) for(int i=(a),_b=(b);i>=_b;--i)
#define FOREACH(e, v) for(auto const &e: v)
#define IN(e, s) (s).find((e)) != (s).end()
#define TRACE(x) cerr << "TRACE(" #x ")" << endl;
#define DEBUG(x) cerr << #x << " = " << (x) << endl;
#define PB push_back
#define ST first
#define ND second
#define ALL(c) (c).begin(), (c).end()
#define SZ(x) ((int)(x).size())
typedef long long LL; typedef unsigned long long ULL; typedef unsigned short UST; typedef unsigned int UINT;
typedef vector<int> VI;
inline  void fix_io() { cin.tie(nullptr); ios::sync_with_stdio(false); }

int main() {
    fix_io();
    UINT n,k;
    UINT c=0;
    cin >> n >> k;
    set<UINT> uq;
    vector<UINT> idx;
    FOR(i,0,n-1) {
        UINT a;
        cin >> a;
        if (c != k) {
            if (uq.find(a) == uq.end()) {
                uq.insert(a);
                idx.PB(i);
                c++;
            }
        }
    }

    if (c < k) {
        cout << "-1\n";
        return 0;
    }

    UINT pos = 1;
    ULL ans = 0;
    FOR(i,1,idx.size()-1) {
        ans += idx[i] - pos;
        pos++;
    }

    cout << ans << '\n';
}