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#include <bits/stdc++.h>
using namespace std;
using LL = long long;
#define FOR(i, l, r) for(int i = (l); i <= (r); ++i)
#define REP(i, n) FOR(i, 0, (n) - 1)
#define ssize(x) int(x.size())
template<class A, class B> auto& operator<<(ostream &o, pair<A, B> p) {
    return o << '(' << p.first << ", " << p.second << ')';
}
template<class T> auto operator<<(ostream &o, T x) -> decltype(x.end(), o) {
    o << '{'; int i = 0; for(auto e : x) o << (", ")+2*!i++ << e; return o << '}';
}
#ifdef DEBUG
#define debug(x...) cerr << "[" #x "]: ", [](auto... $) {((cerr << $ << "; "), ...); }(x), cerr << '\n'
#else
#define debug(...) {}
#endif

int main() {
    cin.tie(0)->sync_with_stdio(0);

    int n, k;
    cin >> n >> k;

    vector<int> a(n);
    REP(i, n)
        cin >> a[i];

    vector<int> used(n + 1);
    int ile = 0;
    LL ans = 0;
    REP(i, n) {
        if(used[a[i]]++ == 0) {
            ans += i - ile;
            if(++ile == k) break;
        }
    }

    if(ile != k) 
        cout << "-1\n";
    else
        cout << ans << "\n";
}