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#include <iostream>
#include <set>
#include <vector>

int main() {
    using namespace std;
    using ll = long long;
    ll n, k, c = 0, result = 0;
    cin >> n >> k;
    vector<int> A(n);
    for(int i = 0; i < n; i++) cin >> A[i];
    set<int> B;
    for(int i = 0; i < n && B.size() < k; i++) {
        if(B.find(A[i]) == B.end()) {
            B.insert(A[i]);
            result += c;
        } else {
            c += 1;
        }
    }
    cout << (B.size() == k ? result : -1) << std::endl;
    return 0;
}