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// #include <bits/stdc++.h>
#define FAST ios_base::sync_with_stdio(false), cin.tie(0), cout.tie(0);
#include <iostream>
#include <vector>
#include <map>
using namespace std;

void solve()
{
}

int main()
{
    FAST int64_t n, k, v;
    cin >> n >> k;

    vector<int> occ(n + 1, -1);
    int64_t needed = 0;
    int64_t steps = 0;
    for (int i = 0; i < k; i++)
    {
        cin >> v;
        if (occ[v] == -1)
        {
            occ[v] = i;
        }
        else
        {
            needed++;
            steps += (k - i);
        }
    }
    if (!needed)
    {
        cout << 0 << endl;
        return 0;
    }

    if (needed > (n - k))
    {
        cout << -1 << endl;
        return 0;
    }
    for (int i = 0; i < (n - k); i++)
    {
        cin >> v;
        if (occ[v] == -1)
        {
            occ[v] = i;
            needed--;
            steps += i;
            if (!needed)
            {
                cout << steps << endl;
                return 0;
            }
        }
    }
    cout << -1 << endl;
}