#include <iostream>
#include <set>
#include <algorithm>

int main() {
    std::ios::sync_with_stdio(false);
    int k, n;
    std::cin >> n >> k;
    std::set<int> v;
    int res = 0;
    int count = 0;
    bool end = false;
    for (int i = 0 ; i < n ; ++i) {
        int x;
        std::cin >> x;
        if (!end && (v.find(x) == v.end())) {
            res += i - count;
            count += 1;
            v.insert(x);
        }
        if (count == k) end = true;
    }
    if (end) {
        std::cout << res;
    } else {
        std::cout << -1;
    }

    return 0;
}