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#include <bits/stdc++.h>
using namespace std;

#define sim template <class c
#define ris return *this
#define dor > debug &operator<<
#define eni(x)                                                                    \
    sim > typename enable_if<sizeof dud<c>(0) x 1, debug &>::type operator<<(c i) \
    {
sim > struct rge
{
    c b, e;
};
sim > rge<c> range(c i, c j) { return rge<c>{i, j}; }
sim > auto dud(c *x) -> decltype(cerr << *x, 0);
sim > char dud(...);
struct debug
{
#ifdef XOX
    ~debug()
    {
        cerr << endl;
    }
    eni(!=) cerr << boolalpha << i;
    ris;
} eni(==) ris << range(begin(i), end(i));
}
sim, class b dor(pair<b, c> d)
{
    ris << "" << d.first << " --> " << d.second << "";
}
sim dor(rge<c> d)
{
    *this << "[";
    for (auto it = d.b; it != d.e; ++it)
        *this << ", " + 2 * (it == d.b) << *it;
    ris << "]";
}
#else
    sim dor(const c &)
    {
        ris;
    }
#endif
}
;

#define imie(...) "" << #__VA_ARGS__ " = " << (__VA_ARGS__) << ", "
#define pb push_back
#define eb emplace_back
#define lb lower_bound
#define ub upper_bound
#define f first
#define s second

#define sz(x) (x).size()
#define all(x) (x).begin(), (x).end()
#define vt vector

using ll = long long;
using pi = pair<int, int>;
using pll = pair<ll, ll>;

using vi = vt<int>;
using vvi = vt<vi>;
using vll = vt<ll>;
using vvll = vt<vll>;
using vs = vt<string>;
using vpi = vt<pi>;
using vvpi = vt<vpi>;
using vpll = vt<pll>;
using vvpll = vt<vpll>;

const int mod = 1e9 + 7;
const int nax = 1e6 + 9;
int a[nax];
int cnt[nax];
set<int> st;
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie();
    int n, k;
    cin >> n >> k;
    for (int i = 1; i <= n; i++)
        cin >> a[i];
    for (int i = 1; i <= k; i++)
        cnt[a[i]]++;
    int tomove = 0;
    int new_slot = k;
    ll ans = 0;
    for (int i = k; i >= 1; i--)
    {
        if (cnt[a[i]] > 1)
        {
            tomove++;
            ans += (new_slot - i);
            new_slot--;
            cnt[a[i]]--;
        }
    }
    new_slot = k - tomove + 1;
    for (int i = k + 1; i <= n; i++)
    {
        if (tomove == 0)
            break;
        if (cnt[a[i]] == 0)
        {
            cnt[a[i]]++;
            ans += (i - new_slot);
            new_slot++;
            tomove--;
        }
    }
    if (tomove != 0)
        cout << -1;
    else
        cout << ans;
}