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#include <cstdio>
#include <iostream>
#include <vector>

using namespace std;

int n,k;

int present[500001] = {0};
int arr[500001];

vector <int> positions;

int main(){
	scanf("%d %d", &n, &k);
	int d;
	for(int i=0;i<n;++i){
		scanf("%d", &arr[i]);
		if(present[arr[i]] != 1){
			present[arr[i]] = 1;
			positions.push_back(i);
		}
	}

	

	if(positions.size() < k){
		printf("-1");
	}else{
		long long int sum=0;
		for(int i=0;i<k;++i){
			sum += positions[i]-i;
		}

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