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#include <bits/stdc++.h>

using namespace std;

int a[500500];
unordered_set<int> unqe;
vector<int> lft;
vector<int> rgt;

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

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

    for(int i = 1; i <= n; i++) {
        cin >> a[i];
    }

    for(int i = 1; i <= k; i++) {
        if(unqe.find(a[i]) != unqe.end()) {
            lft.push_back(i);
        }
        unqe.insert(a[i]);
    }

    for(int i = k + 1; i <= n; i++) {
        if(unqe.find(a[i]) == unqe.end()) {
            rgt.push_back(i);
            unqe.insert(a[i]);
        }
    }

    if(rgt.size() < lft.size()) {
        cout << -1 << "\n";
        return 0;
    }
    
    long long r = 0;
    for(int i = 0; i < lft.size(); i++) {
        r += rgt[i] - lft[i];
    }
    cout << r << "\n";

    return 0;
}