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#include<iostream>

using namespace std;

#define MAXN 500000

int n, k;
// vector<int> seq;
bool seen[MAXN];

int main() {

   cin >> n >> k;
   
   unsigned long long ans = 0;
   int types = 0;

   for(int i = 0; i < n; i++) {
      
      int botno;
      cin >> botno;
      // seq.push_back(botno);
      if(seen[botno]) {
         continue;
      }
      seen[botno] = true;
      ans += i - types;
      if(++types == k) {
         cout << ans;
         cout.flush();
         return 0;
      }
   }

   cout << -1;
   cout.flush();

}