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#include <iostream>
#include <vector>

using namespace std;

bool VectSearch(vector<int> vect, int k){
	
	//cout<<vect.size()<<'\n';
	
	for(int i=0; i<vect.size(); i++){
		if(k==vect[i]) return false;
	}
	
	return true;
}

int main(){
	
	int n, k, answer=0, s;
	vector<int> V;
	cin>>n>>k;
	
	for(int i=0; i<n; i++){
		cin>>s;
		if(VectSearch(V, s)){
			answer += i - V.size();
			V.push_back(s);
			
		}
		if(V.size()>k) break;
		
		//for(int j=0; j<V.size(); j++){
		//	cout<<V[j]<<'\n';
		//}
		//cout<<answer<<'\n';
	}
	
	if(V.size()<k) cout<<-1;
	
	else cout<<answer;
	
	return 0;
}