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#include <bits/stdc++.h>

using namespace std;

int main() {
    std::ios_base::sync_with_stdio(false);

    long long n, k;

    cin >> n >> k;
    vector<bool> alreadyWas;
    for(int i = 0; i <= n; i++)
        alreadyWas.push_back(false);

    long long Pk;
    int howManyDifferent = 0;
    long long timeNeeded;
    long long sumOfWrongBottles = 0;

    for(int i = 0; i < n; i++) {
        int nextBottle;
        cin >> nextBottle;

        if(alreadyWas[nextBottle]) {
            sumOfWrongBottles += i;
        }
        else {
            alreadyWas[nextBottle] = true;
            howManyDifferent++;
            if(howManyDifferent == k){
                Pk = i;
                break;
            }
        }
    }

    if(howManyDifferent == k) {
        timeNeeded = (k + Pk)*(Pk - k + 1)/2 - sumOfWrongBottles;
        cout << timeNeeded;
    }
    else {
        cout << -1;
    }


    return 0;
}