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#include <cstdio>
#include <vector>
#include <cstdlib>

// #define DEBUG_M
#ifdef DEBUG_M
#define DEBUGF(...) printf(stderr, __VA_ARGS__)
#define DEBUG(expr) expr
#else
#define DEBUGF(...)
#define DEBUG(expr)
#endif

using namespace std;

int main() {
	int n;
	int k;
	scanf(" %d %d", &n, &k);
	
	std::vector<int> bottle(n);
	std::vector<int> lookup(n, 0);
	int lkIdx = 1;
	
	for (int i = 0; i < n; i++) {
		int tmp;
		scanf(" %d", &tmp);
		if (!lookup[tmp-1]) {
			lookup[tmp-1] = lkIdx++;
		}
		bottle[i] = lookup[tmp-1];
	}

	if (k == 1) {
		puts("0");
		return 0;
	}
	
	if (lkIdx <= k) {
		puts("-1");
		return 0;
	}

	int lastIn = 1;	
	long long count = ((long long) k) * (k-1);
	count >>= 1; // Epic bind moment B)
	count = -count;
	
	for (int i = 1; lastIn < k; i++) { // && i < n ale wyżej to gwarantujemy
		if (bottle[i] > lastIn) {
			count += i;
			lastIn++;
		}
	}
	
	if (lastIn == k) {
		printf("%lld\n", count);
	} else {
		puts("-1");
	}
}