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#include <bits/stdc++.h>

using namespace std;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    int numberOfBottles, kLeft;
    cin >> numberOfBottles >> kLeft;
    vector<bool> visited(numberOfBottles + 1);
    long long answer = 0;
    int lastPlace = 0;
    for (int i = 0; i < numberOfBottles; i++) {
        int value;
        cin >> value;
        if (visited[value]) {
            continue;
        }
        visited[value] = true;
        answer += (i - lastPlace++);
        if (lastPlace == kLeft) {
            break;
        }
    }
    cout << (lastPlace == kLeft ? answer : -1);// << endl;
}