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#include <bits/stdc++.h>

using namespace std;

#define ll long long

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(NULL);

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

    int a[n];
    set<int> t;
    for (int i=0; i<n; ++i) {
        cin>>a[i];
        t.insert(a[i]);
    }

    if ((int)t.size() < k) {
        cout<<-1<<endl;
        return 0;
    }

    set<int> s;
    int r=0;

    ll c = 0;
    for (int i=0; i<k; ++i) {
        r = max(r, i);

        if (s.find(a[i]) == s.end()) {
            s.insert(a[i]);
            continue;
        }

        for (; r<n; ++r) {
            if (s.find(a[r]) == s.end()) {
                c += (ll)(r-i);
                s.insert(a[r]);
                ++r;
                break;
            }
        }
    }

    cout<<c<<endl;
}