//Author: Piotr Zielinski
#include<bits/stdc++.h>
using namespace std;

template<class C> C reversed(C c) {reverse(c.begin(),c.end()); return c;}
#define rep(i, n) for(int i=0;i<(int)(n);i++)
#define all(X) (X).begin(), (X).end()
#define mp make_pair
#define st first
#define nd second
typedef long long ll;
typedef pair<int,int> pii;

int32_t main(){
    ios::sync_with_stdio(false);
    int n, k;
    cin >> n >> k;

    vector<bool> isData(n + 1);
    int data;
    ll used = 0, res = 0;

    for(int i = 0;i < n && k > 0;++i) {
        cin >> data;
        if(isData[data]) {
            ++used;
        }
        else {
            isData[data] = true;
            --k;
            res += used;
        }
    }

    if(k > 0)
        cout << "-1\n";
    else
        cout << res << "\n";

    return 0;
}