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#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
	ios_base::sync_with_stdio(false);
	int n, k;
	cin >> n >> k;
	vector<int> data(n+1, -1);
	for(int i = 0; i <= n; ++i) {
		int x;
		cin >> x;
		if(data[x] == -1) {
			data[x] = i;
		}
	}
	vector<int> vec;
	for(int i = 0; i <= n; ++i) {
		if(data[i] != -1) {
			vec.push_back(data[i]);
		}
	}
	
	sort(vec.begin(), vec.end());
	
	int res = 0;
	for(int i = 0; i < vec.size() && i < n; ++i) {
		if(vec[i] != i) {
			res += vec[i] - i;
		}
	}
	
	if(vec.size() < k) {
		res = -1;
	}
	
	cout << res << endl;
	
	return 0;
}