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#include <cstdio>
#include <unordered_set>
using namespace std;

int nextInt() { int n; scanf("%d", &n); return n; }

long long solve() {
	int n = nextInt();
	int k = nextInt();
	unordered_set<int> data;

	long long res = 0 - k;
	res *= k - 1;
	res /= 2;

	for (int i = 0; i < n; ++i) {
		int x = nextInt();
		if (data.count(x) > 0) continue;
		res += i;
		data.insert(x);
		if (data.size() == k) return res;
	}

	return -1;
}

int main() {
	long long res = solve();
	printf("%lld\n", res);
	return 0;
}