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

typedef long long LL;
typedef pair<int, int> PII;
typedef pair<LL, LL> PLL;
typedef vector<int> VI;

const int INF = 1000000009;
const LL LINF = 1000000000000000009LL;

#define FOR(i, b, e) for (int i = b; i <= e; ++i)
#define FORD(i, b, e) for (int i = b; i >= e; --i)
#define REP(i, n) FOR (i, 0, n - 1)
#define REV(i, n) FORD (i, n - 1, 0)
#define PB push_back
#define PP pop_back
#define MP make_pair
#define MT make_tuple
#define ST first
#define ND second
#define SZ(c) (int)(c).size()
#define ALL(c) (c).begin(), (c).end()
#define UNIQ(c) (c).erase(unique(ALL(c)), (c).end());

// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/hash_policy.hpp>
// using namespace __gnu_pbds;
// typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set; /*find_by_order() order_of_key()*/

template <typename T, typename S>
ostream &operator<<(ostream &out, const pair<T, S> &v)
{
    return (out << "(" << v.ST << ", " << v.ND << ")");
}

#define DEBUG(s) s
#ifdef local_comp
#define PRINT(V) cout << "#" << #V << ": " << V << endl;
#else
#define PRINT(v)
#endif

const int N = 500005;

int n, k;

int tab[N];
bool vis[N];
VI buf;
LL res;

int main()
{
#ifndef local_comp
    ios::sync_with_stdio(0);
    cin.tie(0);
#endif

    cin >> n >> k;
    FOR (i, 1, n)
        cin >> tab[i];

    FOR (i, 1, n)
    {
        if (!vis[tab[i]])
        {
            buf.PB(i);
            vis[tab[i]] = true;
        }
    }

    if (SZ(buf) < k)
    {
        cout << "-1";
        return 0;
    }

    REP (i, k)
    {
        res += buf[i] - 1 - i;
    }

    cout << res;

    return 0;
}