#include <bits/stdc++.h>

using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int n, k;
    cin >> n >> k;
    
    vector<int> v(n);
    for (int &i : v)
        cin >> i;
    
    vector<int> fpos(n, n);
    for (int i = n - 1; i >= 0; --i)
        fpos[v[i] - 1] = i;
    sort(fpos.begin(), fpos.end());
    if (fpos[k - 1] == n) {
        cout << "-1\n";
        return 0;
    }

    long long ans = 0;
    for (int i = 0; i < k; ++i)
        ans += (long long)(fpos[i] - i);
    cout << ans << '\n';
}