#include <bits/stdc++.h>
using namespace std;
int mp[28];
bool fl[28];
int main() {
  int n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  memset(mp, 0, sizeof(mp));
  for (int i = 0; i < s.size(); i++) {
    mp[s[i] - 'a' + 1]++;
  }
  int cnt = 0;
  int ans = 0;
  for (int i = 1; i <= 26; i++) {
    if (mp[i] != 0 && fl[i - 1] == 0) {
      ans += i;
      cnt++;
      fl[i] = 1;
    }
    if (cnt >= k) break;
  }
  if (cnt == k)
    cout << ans << endl;
  else
    cout << "-1" << endl;
}
