#include <iostream>
#include <vector>
#include <map>

using namespace std;

int getNearest(map<int, int> *m) {
    int lowestkey = (*m).begin()->first;
    int lowest = (*m).begin()->second;
    for(map<int, int>::iterator it = (*m).begin(); it != (*m).end(); ++it) {
        int x = it->second;
        if(x < lowest) {
            lowest = x;
            lowestkey = it->first;
        }
    }
    (*m).erase(lowestkey);
    return lowest;
}

int main() {
    int n, k;
    cin >> n >> k;

    vector<int> data;
    map<int, int> type_map;
    for(int i = 0; i < n; i++) {
        int x;
        cin >> x;
        data.push_back(x);
        if(type_map.count(x) == 0) type_map[x] = i;
    }

    if(type_map.size() < k) {
        cout << -1;
        return 0;
    }

    int count = 0;
    for(int i = 0; i < k; i++) {
        for(int j = getNearest(&type_map); j > i; j--) {
            int x = data[j];
            data[j] = data[j-1];
            data[j-1] = x;
            count++;
        }
    }

    cout << count;

    return 0;
}