#include<iostream>
#include<vector>
#include<algorithm>
#include<unordered_set>

using namespace std;

int main(){
    int n, k;
    cin >> n >> k;
    vector<int> bottles(n);
    for(int i=0;i<n;i++){
        cin >> bottles[i];
    }
    unordered_set<int> seen;
    long long count = 0;
    for(int i=0;i<n;i++){
        if(seen.find(bottles[i]) == seen.end()){
            seen.insert(bottles[i]);
            count += (i - seen.size() + 1);
        }
        if(seen.size() == k){
            break;
        }
    }
    if(seen.size() < k){
        cout << -1 << endl;
        return 0;
    }
    cout << count << endl;
}