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#include <bits/stdc++.h>

using namespace std;
typedef long long LL;

int main() {
    ios_base::sync_with_stdio(false);
    int n, k, a;
    cin >> n >> k;

    unordered_set<int> diffset;
    int lastdiff = 0;
    LL ct = 0;
    for(int i = 0; i < n && lastdiff < k; ++i) {
        cin >> a;
        if(diffset.count(a) > 0) {
            continue;
        } else {
            ct += i-lastdiff;
            lastdiff++;
            diffset.insert(a);
        }
    }
    if(lastdiff < k) cout << -1 << endl;
    else cout << ct << endl;
}