#include <iostream>
#include <set>
#include <algorithm>
using namespace std;

int foo(int* start, int* end) {
	int ret = 0;
	for(int* i = start+1; i < end; ++i) {
		if(*start != *i) {
			swap(*start, *i);
			return ret;
		}
		ret++;
	}
	return 0;
}

int main() {
	ios::sync_with_stdio(0);
	cout.tie();
	cin.tie();
	auto* s = new set<int>;
	int n, k, answer = 0;
	cin >> n >> k;
	int arr[n];
	for(int& i : arr) {
		cin >> i;
		s->insert(i);
	}
	if(s->size() < k) {
		cout << "-1\n";
		return 0;
	}
	delete s;
	for(int* i = arr; i < arr+k; ++i) {
		answer += foo(i, arr+n);
	}
	cout << answer << '\n';
}