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#include <vector>
#include <set>
#include <cstdio>

#define ll long long

using namespace std;

ll solve(vector<int>& a, int k) {
  int n = a.size();
  set<int> pastBottles;
  int j = 0;
  ll res = 0;
  for(int i=0;i<k;i++) {
    if(!pastBottles.insert(a[i]).second) {
      if(j <= i) {
        j = i + 1;
      }
      while((j < n) && (!pastBottles.insert(a[j]).second)) j++;
      if(j >= n) {
        return -1;
      }
      res += j - i;
    }
  }
  return res;
}

int main(int argc, char** argv) {
  int n, k;
  scanf("%d%d", &n, &k);
  vector<int> a(n);
  for(int i=0;i<n;i++) {
    scanf("%d",&(a[i]));
  }
  ll res = solve(a, k);
  printf("%lld\n", res);
  return 0;
}