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#include "bits/stdc++.h" // Tomasz Nowak
using namespace std;     // University of Warsaw
using LL = long long;
#define FOR(i, l, r) for(int i = (l); i <= (r); ++i)
#define REP(i, n) FOR(i, 0, (n) - 1)
#define ssize(x) int(x.size())
template<class A, class B> auto& operator<<(ostream &o, pair<A, B> p) {
	return o << '(' << p.first << ", " << p.second << ')';
}
template<class T> auto operator<<(ostream &o, T x) -> decltype(x.end(), o) {
	o << '{'; int i = 0; for(auto e : x) o << (", ")+2*!i++ << e; return o << '}';
}
#ifdef DEBUG
#define debug(x...) cerr << "[" #x "]: ", [](auto... $) {((cerr << $ << "; "), ...); }(x), cerr << '\n'
#else
#define debug(...) {}
#endif

// https://github.com/tonowak/acmlib/blob/master/code/data-structures/fenwick-tree/main.cpp
struct Fenwick {
	vector<int> s;
	Fenwick(int n) : s(n) {}
	void update(int pos, int val) {
		for(; pos < ssize(s); pos |= pos + 1)
			s[pos] += val;
	}
	int query(int pos) {
		int ret = 0;
		for(pos++; pos > 0; pos &= pos - 1)
			ret += s[pos - 1];
		return ret;
	}
};

int main() {
	cin.tie(0)->sync_with_stdio(0);

	int n, k;
	cin >> n >> k;
	vector<int> a(n);
	for(int &ai : a) {
		cin >> ai;
		--ai;
	}

	set<int> on_prefix;
	vector<int> non_dup, dup;
	int p = 0;
	for(; p < n; ++p) {
		if(on_prefix.find(a[p]) == on_prefix.end()) {
			non_dup.emplace_back(a[p]);
			on_prefix.emplace(a[p]);
		}
		else
			dup.emplace_back(a[p]);

		if(ssize(on_prefix) == k)
			break;
	}
	debug(non_dup, dup);
	if(p == n) {
		cout << "-1\n";
		return 0;
	}
	++p;

	vector<int> merged = non_dup;
	for(int x : dup)
		merged.emplace_back(x);
	debug(merged);

	vector<vector<int>> init_pos(n), end_pos(n);
	REP(i, p) {
		init_pos[a[i]].emplace_back(i);
		end_pos[merged[i]].emplace_back(i);
	}
	debug(init_pos, end_pos);

	vector<int> perm(p, -1);
	REP(val, n) {
		assert(ssize(init_pos[val]) == ssize(end_pos[val]));
		REP(i, ssize(init_pos[val]))
			perm[end_pos[val][i]] = init_pos[val][i];
	}
	debug(perm);

	LL answer = 0;
	Fenwick tree(p);
	REP(i, p) {
		answer += i - tree.query(perm[i]);
		tree.update(perm[i], +1);
	}
	cout << answer << '\n';
}