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

typedef long long ll;

int n, k;
ll res;
bool t[500001];

int main()
{
    scanf("%d%d", &n, &k);

    ll ct = 0;

    while(n--)
    {
        int a;

        scanf("%d", &a);

        if(!k)
            continue;

        if(!t[a])
        {
            t[a] = 1;
            k--;

            res += ll(ct);
        }
        else
            ct++;
    }

    if(k)
        printf("-1");
    else
        printf("%lld", res);
}