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#include <bits/stdc++.h>
using namespace std;
bool w_k[1000007];
vector <int> jak_daleko;
vector <int> do_zmiany;
bool poza_k[1000007];
int butelki[1000007];
int main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL);
int n;
int k;
cin >> n >> k;
for (int i = 0; i < n; ++i){
    cin >> butelki[i];
    if (i >= k){
        if (poza_k[butelki[i]] == 0 && w_k[butelki[i]] == 0){
            jak_daleko.push_back(i+1-k);
            poza_k[butelki[i]] = 1;
        }
    }
    else{
        if (w_k[butelki[i]] == 1){
            do_zmiany.push_back(i);
        }
        if (w_k[butelki[i]] == 0){
            w_k[butelki[i]] = 1;
        }
    }
}
long long wynik = 0;
int iter = 0;
for (int i = 0; i < do_zmiany.size(); ++i){
    if (iter >= jak_daleko.size()){
        cout << "-1" << "\n";
        return 0;
    }
    wynik += (k-1)-do_zmiany[i] + (jak_daleko[iter]);
    iter++;
}
cout << wynik << "\n";
return 0;
}