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

long long n, k, mov, a, b;
int t[500007];
int cnt[500007];
vector <int> same;
vector <int> diff;
long long res;

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

    cin >> n >> k;
    for(int i = 1; i <= n; i++){
        cin >> t[i];
        cnt[t[i]]++;
        if(cnt[t[i]] > 1 && i <= k){
            same.push_back(i);
        }
        else if(i > k && cnt[t[i]] == 1){
            diff.push_back(i);
        }
    }

    if(same.size() > diff.size()){
        cout << -1;
        return 0;
    }

    for(int i = 0; i < same.size(); i++){
            //cout << same[i] << "i=" << i << ' ';
            //cout << diff[i] << "i=" << i << '\n';
        a = same[i] + mov;
        b = diff[i];
        mov++;

        res += b - a;
    }

    for(int i = same.size() - 1; i >= 0; i--){
        a = same[i] + mov;
        res += k + i + 1 - a;
        mov--;
    }

    cout << res;

return 0;
}