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#include <bits/stdc++.h>

using namespace std;
#define LL long long

// TODO: Nie wczytuj wszystkich liczb
LL solve(vector<int> &a, int k)
{
    int n = a.size();
    LL moves = 0;
    vector<bool> seen(a.size()+2, false);
    int left = 0,right = 1;
    while (left < k && right < n)
    {
        //printf("Processing left elem a[%d]=%d\n", left, a[left]);
        if (!seen[a[left]])
        {
            seen[a[left]] = true;
            left++;
        }
        else
        {
            while (right < n && seen[a[right]])
            {
                right++;
            }
            if (right == n) break;
            seen[a[right]] = true;
            moves += right - left;
            swap(a[left], a[right]);
            left++;
        }
    }
    if (right == n) return -1;

    return moves;
}

int main() {
    int n,k;
    scanf("%d%d", &n,&k);
    vector<int> a(n, 0);
    for (int i=0; i < n; i++)
    {
        scanf("%d", &a[i]);
    }
    printf("%lld\n", solve(a,k));
}