#include <bits/stdc++.h>
long long dx[8] = {0, 1, 0, -1, 1, 1, -1, -1};
long long dy[8] = {1, 0, -1, 0, -1, 1, 1, -1};
using namespace std;
class pa3 {
 public:
  long long x;
  long long y, z;
  pa3(long long x = 0, long long y = 0, long long z = 0) : x(x), y(y), z(z) {}
  bool operator<(const pa3 &p) const {
    if (x != p.x) return x < p.x;
    if (y != p.y) return y < p.y;
    return z < p.z;
  }
  bool operator>(const pa3 &p) const {
    if (x != p.x) return x > p.x;
    if (y != p.y) return y > p.y;
    return z > p.z;
  }
  bool operator==(const pa3 &p) const {
    return x == p.x && y == p.y && z == p.z;
  }
  bool operator!=(const pa3 &p) const {
    return !(x == p.x && y == p.y && z == p.z);
  }
};
class pa4 {
 public:
  long long x;
  long long y, z, w;
  pa4(long long x = 0, long long y = 0, long long z = 0, long long w = 0)
      : x(x), y(y), z(z), w(w) {}
  bool operator<(const pa4 &p) const {
    if (x != p.x) return x < p.x;
    if (y != p.y) return y < p.y;
    if (z != p.z) return z < p.z;
    return w < p.w;
  }
  bool operator>(const pa4 &p) const {
    if (x != p.x) return x > p.x;
    if (y != p.y) return y > p.y;
    if (z != p.z) return z > p.z;
    return w > p.w;
  }
  bool operator==(const pa4 &p) const {
    return x == p.x && y == p.y && z == p.z && w == p.w;
  }
};
class pa2 {
 public:
  long long x, y;
  pa2(long long x = 0, long long y = 0) : x(x), y(y) {}
  pa2 operator+(pa2 p) { return pa2(x + p.x, y + p.y); }
  pa2 operator-(pa2 p) { return pa2(x - p.x, y - p.y); }
  bool operator<(const pa2 &p) const { return y != p.y ? y < p.y : x < p.x; }
  bool operator>(const pa2 &p) const { return x != p.x ? x < p.x : y < p.y; }
  bool operator==(const pa2 &p) const {
    return abs(x - p.x) == 0 && abs(y - p.y) == 0;
  }
  bool operator!=(const pa2 &p) const {
    return !(abs(x - p.x) == 0 && abs(y - p.y) == 0);
  }
};
string itos(long long i) {
  ostringstream s;
  s << i;
  return s.str();
}
long long gcd(long long v, long long b) {
  if (v > b) return gcd(b, v);
  if (v == b) return b;
  if (b % v == 0) return v;
  return gcd(v, b % v);
}
long long mod;
long long extgcd(long long a, long long b, long long &x, long long &y) {
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  }
  long long d = extgcd(b, a % b, y, x);
  y -= a / b * x;
  return d;
}
pair<long long, long long> operator+(const pair<long long, long long> &l,
                                     const pair<long long, long long> &r) {
  return {l.first + r.first, l.second + r.second};
}
pair<long long, long long> operator-(const pair<long long, long long> &l,
                                     const pair<long long, long long> &r) {
  return {l.first - r.first, l.second - r.second};
}
long long pr[1200010];
long long inv[1200010];
long long beki(long long wa, long long rr, long long warukazu) {
  if (rr == 0) return 1 % warukazu;
  if (rr == 1) return wa % warukazu;
  wa %= warukazu;
  if (rr % 2 == 1)
    return ((long long)beki(wa, rr - 1, warukazu) * (long long)wa) % warukazu;
  long long zx = beki(wa, rr / 2, warukazu);
  return (zx * zx) % warukazu;
}
long long comb(long long nn, long long rr) {
  if (rr < 0 || rr > nn || nn < 0) return 0;
  long long r = pr[nn] * inv[rr];
  r %= mod;
  r *= inv[nn - rr];
  r %= mod;
  return r;
}
void gya(long long ert) {
  pr[0] = 1;
  for (long long i = 1; i <= ert; i++) {
    pr[i] = (pr[i - 1] * i) % mod;
  }
  inv[ert] = beki(pr[ert], mod - 2, mod);
  for (long long i = ert - 1; i >= 0; i--) {
    inv[i] = inv[i + 1] * (i + 1) % mod;
  }
}
long long mae[110][30];
long long dp[110][110];
long long ov[110][110];
long long zen[110];
long long oz[110];
signed main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  long long n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  s = " " + s;
  long long er[26] = {};
  long long gen = 10000000000000ll;
  for (long long i = 1; i <= n; i++) {
    for (long long u = 0; u < 26; u++) mae[i][u] = er[u];
    mae[i][s[i] - 'a'] = er[s[i] - 'a'];
    er[s[i] - 'a'] = i;
  }
  for (long long i = 1; i <= n; i++) {
    for (long long u = 0; u < 26; u++) {
      for (long long j = 1; j <= 100; j++) {
        if (ov[i][j]) continue;
        if (ov[mae[i][u]][j - 1]) {
          ov[i][j] = 1;
          continue;
        }
        dp[i][j] += dp[mae[i][u]][j - 1];
        if (dp[i][j] > gen) {
          ov[i][j] = 1;
        }
      }
    }
    dp[i][1]++;
  }
  zen[0] = 1;
  for (long long i = 0; i < 26; i++) {
    for (long long j = 0; j <= 100; j++) {
      if (oz[j]) continue;
      zen[j] += dp[er[i]][j];
      if (ov[er[i]][j]) oz[j] = 1;
      if (zen[j] > gen) {
        oz[j] = 1;
      }
    }
  }
  long long kosuu = 0;
  bool ok = 0;
  for (long long i = 0; i <= n; i++) {
    if (oz[i]) {
      ok = 1;
      break;
    }
    kosuu += zen[i];
  }
  if (kosuu < k && !ok) {
    cout << -1 << endl;
    return 0;
  }
  long long ans = 0;
  for (long long i = n; i >= 0; i--) {
    if (k == 0) break;
    if (oz[i]) {
      ans += (n - i) * k;
      break;
    }
    long long e = min(k, zen[i]);
    ans += (n - i) * e;
    k -= e;
  }
  cout << ans << endl;
  return 0;
}
