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#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#ifdef LOC
#include "debuglib.h"
#else
#define deb(...)
#define DBP(...)
#endif
using namespace std;
using   ll         = long long;
using   Vi         = vector<int>;
using   Pii        = pair<int, int>;
#define pb           push_back
#define mp           make_pair
#define x            first
#define y            second
#define rep(i, b, e) for (int i = (b); i < (e); i++)
#define each(a, x)   for (auto& a : (x))
#define all(x)       (x).begin(), (x).end()
#define sz(x)        int((x).size())

int main() {
	cin.sync_with_stdio(0); cin.tie(0);
	cout << fixed << setprecision(12);

	int n, k; cin >> n >> k;
	Vi elems(n);
	each(e, elems) cin >> e, e--;

	vector<bool> seen(n);
	ll ans = 0;
	int i = 0;

	rep(j, 0, k) {
		while (i < n && seen[elems[i]]) i++;
		if (i >= n) {
			cout << "-1\n";
			return 0;
		}
		seen[elems[i]] = 1;
		ans += i-j;
	}

	cout << ans << '\n';
	return 0;
}