#include <bits/stdc++.h>
using namespace std;
const long long MOD = 1e9 + 7;
const int INF = 1e9;
const long long IINF = 1e18;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, 1, 0, -1};
const char dir[4] = {'D', 'R', 'U', 'L'};
template <typename T>
istream &operator>>(istream &is, vector<T> &v) {
  for (T &x : v) is >> x;
  return is;
}
template <typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < v.size(); ++i) {
    os << v[i] << (i + 1 == v.size() ? '\n' : ' ');
  }
  return os;
}
template <typename T, typename U>
ostream &operator<<(ostream &os, const pair<T, U> &p) {
  cout << '(' << p.first << ',' << p.second << ')';
  return os;
}
template <class T>
inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
template <class T>
inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n;
  long long k;
  cin >> n >> k;
  string S;
  cin >> S;
  vector<vector<int>> nxt(n + 1, vector<int>(26, n));
  for (int i = n - 1; i >= 0; --i) {
    for (int j = 0; j < 26; ++j) nxt[i][j] = nxt[i + 1][j];
    nxt[i][S[i] - 'a'] = i;
  }
  vector<vector<long long>> dp(n + 1, vector<long long>(n + 1, 0));
  dp[0][0] = 1;
  for (int i = 0; i < n; ++i) {
    for (int j = 0; j < n; ++j) {
      for (int k = 0; k < 26; ++k) {
        if (n <= nxt[i][k]) continue;
        dp[nxt[i][k] + 1][j + 1] += dp[i][j];
      }
    }
  }
  long long ans = 0;
  for (int j = n; j >= 0; --j) {
    long long sum = 0;
    for (int i = 0; i <= n; ++i) sum += dp[i][j];
    ans += min(sum, k) * (n - j);
    k -= min(sum, k);
    if (k == 0) break;
  }
  cout << (0 < k ? -1 : ans) << '\n';
}
