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#include<bits/stdc++.h>

#define maxn 500010

using namespace std;

int tab[maxn];
bool was[maxn];

bool por(int a, int b) {
    return a > b;
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(NULL);
    int n,k;
    cin >> n >> k;
    for (int i = 0; i < n; ++i) {
        cin >> tab[i];
    }
    long long count = 0;
    long long wynik = 0;
    for (int i = 0; i < n && k != 0; ++i) {
        if (was[tab[i]])
            count++;
        else {
            wynik += count;
            was[tab[i]] = true;
            --k;
        }
    }
    if (k != 0) {
        cout << "-1";
    }
    else {
        cout << wynik;
    }
}