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#include<bits/stdc++.h>
using namespace std;

int main(){
	int n, m;
	cin >> n >> m;
	vector<int> a;
	bool b[n];
	for(int i = 0 ;i < n; i++){
		int temp;
		cin >> temp;
		a.push_back(temp);
		b[i]=0;
	}
	int fin = 0;
	int c=0;
	for(int i = 0; i<m; i++){
		if(!b[a[i]-1]){
			b[a[i]-1]=1;
			fin+=i-c;
			c++;
		}
	}
	for(int i = m ; i< n&&c!=m; i++){
		if(!b[a[i]-1]){
			b[a[i]-1]=1;
			fin+=i-c;
			c++;
		}
	}
	if(c!=m) cout << -1;
	else cout << fin;
	return 0;
}