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#include <bits/stdc++.h>
using namespace std;
#define IOS                       \
    ios_base::sync_with_stdio(0); \
    cin.tie(0);                   \
    cout.tie(0);
#define rep(i, a, n) for (int i = a; i <= n; i++)
#define repV(i, a, n) for (int i = n; i >= a; --i)
#define st first
#define nd second
#define pb push_back
#define mp make_pair
#define all(a) a.begin(), a.end()
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
typedef pair<int, int> pii;
constexpr int M = 5e5 + 5;
int t[M], n, k, pop = 1, ans, idx = 1;
bool check, odw[M];

void solve()
{
    cin >> n >> k;
    rep(i, 1, n)
    {
        cin >> t[i];
    }
    while (idx <= k)
    {
        check = 0;
        rep(i, pop, n)
        {
            if (!odw[t[i]])
            {
                check = 1;
                pop = i;
                odw[t[i]] = 1;
                ans += (i - idx);
                idx++;
            }
        }
        if (!check)
        {
            cout << "-1\n";
            return;
        }
    }
    cout << ans << "\n";
}

int main()
{
    IOS
    solve();
}