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#include<bits/stdc++.h>
#define ll long long
#define pii pair<int, int>
#define st first
#define nd second
#define turbo ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define pb push_back
#define vi vector<int>
#define vvi vector<vi>
#define qi queue<int>
#define ld long double

using namespace std;

/*---------------------------------------------------------///CODE///---------------------------------------------------------*/

int main()
{
    turbo

    int n, k; cin >> n >> k;

    vi v(n);
    set<int> S;
    vector<bool> vis(n + 10, false);

    for(int i = 0; i < n; i++)
    {
        int x; cin >> x;
        S.insert(x);
        v[i] = x;
    }

    if(S.size() < k)
    {
        cout << "-1\n";
        return 0;
    }

    int curr = 0;
    ll res = 0;

    for(int i = 0; i < n; i++)
        if(!vis[v[i]] && k > 0)
        {
            res += i - curr;
            curr ++;
            k --;
            vis[v[i]] = true;
        }

    cout << res;

    return 0;
}