#include <bits/stdc++.h>
using namespace std;
const int MAXN = 100 + 7;
int n, k;
string second;
string s2;
string s1;
int main() {
  cin >> n >> k;
  cin >> second;
  sort(second.begin(), second.end());
  for (int i = 0; i < n; i++) {
    if (second[i] != second[i + 1]) {
      s2 += second[i];
    }
  }
  second = "";
  second += s2;
  sort(second.begin(), second.end());
  for (int i = 0; i < second.size(); i++) {
    s1 += second[i];
    if (second[i] + 1 == second[i + 1] || second[i] == second[i + 1]) {
      i++;
    }
  }
  if (s1.size() >= k) {
    int cnt = 0;
    for (int i = 0; i < k; i++) {
      cnt += s1[i] - 'a' + 1;
    }
    cout << cnt;
  } else {
    cout << -1;
  }
}
