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#include <bits/stdc++.h>

const int MAXN = 1000005;
int used[MAXN];

int main() {
	int n, k;
	scanf("%d%d", &n, &k);
	
	int a;
	int cnt = 0;
	long long res = 0;
	
	for (int i=0; i<n; i++) {
		scanf("%d", &a);
		if (used[a] != 0)
			continue;
		
		used[a] = 1;
		res += i-cnt;
		cnt++;
		
		if (cnt == k) {
			printf("%lld\n", res);
			return 0;
		}
	}
	
	printf("-1\n");
	return 0;
}