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#include <cstdio>
#include <algorithm>
#include <vector>
#include <set>

using namespace std;

typedef long long LL;

#define M 500007

int n, k;
int t[M];
bool exists[M];

int main() {
	scanf("%d%d", &n, &k);
	for (int i=0; i<n; i++) scanf("%d", &t[i]);

	int last=0, different=0;
	LL result=0;
	for (; last<n && different<k; last++) {
		if (exists[t[last]]) result+=(LL)(k-different);
		else exists[t[last]]=true, different++;
	}

	if (different<k) {
		printf("-1\n");
		return 0;
	}

	printf("%lld\n", result);
	return 0;
}