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#include <bits/stdc++.h>

using namespace std;

bitset<500007> unq;
int unqs[500007];
int unqs_c=0;
unsigned long long res = 0;

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    int n, k, a, j;
    cin >> n >> k;
    for(int i = 0; i < n; i++) {
        cin >> a;
        if (!unq[a]) {
            //cout << a << " is unq" << endl;
            unqs[unqs_c] = i;
            //cout << unqs[unqs_c] << " " << i << endl;
            unqs_c++;
            unq[a] = true;
        }
    }
    if (unqs_c < k) {
        cout << "-1";
        return 0;
    }
    /*
    for(int i = 0; i < unqs_c; i++) {
        cout << unqs[i] << " ";
    }
    */
    j=0;
    for(int i = 0; i < k; i++) {
        res += unqs[i]-i;
    }
    cout << res << endl;
    return 0;
}