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#include <iostream>
#include <vector>
#include <algorithm>
#include <set>
using namespace std;
int n,k;
vector <int> v;
set<int> used;

int main(){
	scanf("%d%d",&n,&k);
	for (int i =0; i < n; i++){
		int x;
		scanf("%d", &x);
		v.push_back(x);
	}



	int score = 0;
	int size = 0;
	for (int i = 0; i < n; i++){
		int x = v[i];
		if ( used.find(x) == used.end()){
			score+= i-size;
			used.insert(x);
			size++;
		}
		// printf("i = %d, scoer = %d\n", i, score);
		if ( size == k ){
			printf("%d\n", score);
			return 0;
		}
	}
	
	printf("%d\n", -1);


}