#include <bits/stdc++.h>
#pragma GCC optimize("Ofast,unroll-loops,O2")
#pragma GCC target( \
    "sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
using namespace std;
void itval(istream_iterator<string> it) {}
template <typename T, typename... Args>
void itval(istream_iterator<string> it, T a, Args... args) {
  cerr << *it << " = " << a << endl;
  itval(++it, args...);
}
const long long int MOD = 1e9 + 7;
template <typename T>
inline void print(T x) {
  cout << x << "\n";
}
template <typename T>
inline void printvec(T x) {
  for (auto a : x) cout << a << ' ';
  cout << '\n';
}
struct custom {
  bool operator()(const pair<long long int, long long int> &p1,
                  const pair<long long int, long long int> &p2) const {
    return p1.first > p2.first;
  }
};
long long int get_pow(long long int a, long long int b, long long int M = MOD) {
  long long int res = 1;
  while (b) {
    if (b & 1) res = (res * a) % M;
    a = (a * a) % M;
    b >>= 1;
  }
  return res;
}
const long long int N = 1e3 + 5, inf = 1e14;
long long int n, k;
string s;
long long int calc(long long int x) {
  vector<long long> v1(n, 0), v2(n, 0), v3(26, 0);
  v2[n - 1] = 1;
  v3[s.back() - 'a'] = 1;
  for (int j = n - 2; j >= 0; j--) {
    v2[j] = v2[j + 1];
    if (!v3[s[j] - 'a']) {
      v2[j]++;
      v3[s[j] - 'a'] = 1;
    }
  }
  for (long long int i = (long long int)1; i < (long long int)(x); i++) {
    fill(v3.begin(), v3.end(), 0);
    v1[n - 1] = 0;
    for (int j = n - 2; j >= 0; j--) {
      v1[j] = v1[j + 1];
      v1[j] += v2[j + 1] - v3[s[j] - 'a'];
      v1[j] = min(v1[j], inf);
      v3[s[j] - 'a'] = v2[j + 1];
    }
    v2 = v1;
  }
  return v2[0];
}
void solve() {
  cin >> n >> k;
  cin >> s;
  vector<long long> v(n);
  v[0] = 1;
  for (long long int i = (long long int)1; i < (long long int)(n + 1); i++)
    v[i] = calc(i);
  long long int ans = 0, w;
  for (long long int j = n; j >= 0; j--) {
    w = min(k, v[j]);
    ans += w * (n - j);
    k -= w;
  }
  if (k)
    cout << -1;
  else
    cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int test = 1;
  clock_t z = clock();
  for (long long int tes = (long long int)0; tes < (long long int)(test);
       tes++) {
    solve();
  }
  fprintf(stderr, "Total Time:%.4f\n", (double)(clock() - z) / CLOCKS_PER_SEC),
      fflush(stderr);
  return 0;
}
