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#include <iostream>
#include <vector>
#include <set>

using namespace std;

void C() {
    vector<int> data(121,0);
    int n, k, a;
    int kUsed = 0;

    cin >> n >> k;
    
    for (int i = 0; i < n; i++){
        cin >> a;
        data[a]++;
    }

    int i = 120;

    while (k > kUsed) {
        kUsed += data[i];
        i--;
        if (i < 0)break;
    }
    cout << kUsed;
}

void B() {
    int n, k;
    cin >> n >> k;
    vector<int> bottles(n);
    set<int> usedB;

    for (int i = 0; i < n; i++){
        cin >> bottles[i];
    }
    int bottlesPlaced = 0;
    int a;
    int time = 0;
    for (int i = 0; i < n; i++){
        a = bottles[i];
        if (usedB.find(a) == usedB.end()) {
            time += (i - bottlesPlaced);
            bottlesPlaced++;
            usedB.insert(a);
        }
        if (bottlesPlaced == k) {
            cout << time;
            return;
        }
    }

    cout << "-1";
}

int main()
{
    B();
    return 0;
}