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#include <bits/stdc++.h>

#define ll long long
#define str string
#define pii pair<int, int>
#define pll pair<ll, ll>
#define fi first
#define se second

#define vc vector<char>
#define vvc vector<vc>
#define vi vector<int>
#define vvi vector<vi>
#define vvvi vector<vvi>
#define vvvvi vector<vvvi>
#define vll vector<ll>
#define vvll vector<vll>
#define vvvll vector<vvll>
#define vvvvll vector<vvvll>
#define vs vector<str>
#define vvs vector<vs>
#define vpii vector<pii>
#define vvpii vector<vpii>
#define vpll vector<pll>
#define vvpll vector<vpll>
#define vb vector<bool>
#define vvb vector<vb>
#define rep(i, a, b) for (int i = (a); i < int(b); i++)
#define repi(i, a, b) for (int i = (a); i <= int(b); i++)

using namespace std;


void solve() {
    int n, k;
    cin >> n >> k;
    vi a(n);
    rep(i, 0, n) {
        cin >> a[i];
        a[i]--;
    }

    vb seen(n, false);
    vi is;
    vi js;
    int numseen = 0;

    rep(i, 0, n) {
        if (i < k && seen[a[i]]) {
            is.push_back(i);
        }
        if (i >= k && !seen[a[i]]) {
            js.push_back(i);
        }

        if (!seen[a[i]]) {
            seen[a[i]] = true;
            numseen++;
        }
    }

    if (numseen < k) {
        cout << -1;
        return;
    }

    reverse(is.begin(), is.end());
    ll sum = 0;
    rep(c, 0, is.size()) {
        int i = is[c];
        sum += (k - 1) - c - i;
    }

    rep(c, 0, is.size()) {
        int j = js[c]; // go to k + c
        sum += j - (k + c);
        sum += is.size();
    }

    cout << sum;
}


int main() {

    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    bool _multipleTestCases = false;

    if (_multipleTestCases) {
        ll t; cin >> t;
        while (t--)
            solve();
    }
    else {
        solve();
    }

    return 0;
}