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#include <bits/stdc++.h>
#include <iostream>
#include <vector>
#include <map>

#define MAX_SIZE 500001

using namespace std;

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

    int n, k;

    cin >> n;
    cin >> k;

    vector<int> bottles(n);

    for (int i = 0; i < n; i++) {
        cin >> bottles[i];
    }

    // Algorithm

    vector<int> duplicates(MAX_SIZE, 0);
    vector<int> duplicates_until_k;
    vector<int> unique_after_k;

    for (int i = 0; i < k; i++) {
        if (duplicates[bottles[i]] != 0) {
            duplicates_until_k.push_back(i);
            //cout << "d(" << i << ", " << bottles[i] << ") ";
        } 
        duplicates[bottles[i]] = 1;
    }
    //cout << endl;
    for (int i = k; i < n; i++) {
        if (duplicates[bottles[i]] == 0) {
            unique_after_k.push_back(i);
            //cout << "u(" << i << ", " << bottles[i] << ") ";
        } 
        duplicates[bottles[i]] = 1;
    }

    if (duplicates_until_k.size() > unique_after_k.size()) {
        cout << -1;
    } else {
        long result = 0;
        for (int i = duplicates_until_k.size() - 1, j = 0; i >= 0; i--, j++) {
            //cout << "r(" << result << "," << duplicates_until_k[i] << ", " << (k - 1 - j) << ") ";
            result += ((k - 1 - j) - duplicates_until_k[i]);            
            result += (unique_after_k[j] - (k - 1 - j));
        }
        cout << result;
    }

    return 0;
}