#include <bits/stdc++.h>
using namespace std;
int solve(void) {
  int n, s;
  cin >> n >> s;
  set<int> avail;
  char c;
  for (int i = 0; i < n; i++) {
    cin >> c;
    avail.insert((int)c - 'a' + 1);
  }
  int cnt = 1;
  int ans = *avail.begin();
  int prev = ans;
  avail.erase(avail.begin());
  while (cnt < s) {
    if (avail.empty()) return -1;
    while (*avail.begin() < prev + 2) {
      avail.erase(avail.begin());
      if (avail.empty()) return -1;
    }
    prev = *avail.begin();
    ans += prev;
    avail.erase(avail.begin());
    cnt++;
  }
  return ans;
}
int main(void) { cout << solve() << endl; }
