#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
#define ll long long
#define OO 2'000'000'000
#define ull unsigned long long
#define nl '\n'
#define sz(x) (ll)(x.size())
#define all(x) x.begin(),x.end()
#define rall(s)  s.rbegin(), s.rend()
#define getline(s) getline(cin>>ws,s)
#define ceill(n, m) (((n) / (m)) + ((n) % (m) ? 1 : 0))
#define pi  3.141592653589793
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
#define multi_ordered_set tree<int, null_type,less_equal<int>, rb_tree_tag,tree_order_statistics_node_update>


void Fast_IO(){
ios_base::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
// freopen("filename.in", "r", stdin);
// freopen("filename.txt", "w", stdout);
#ifndef ONLINE_JUDGE
freopen("input.txt", "r", stdin), freopen("output.txt", "w", stdout);
#endif
}




int dx[] = { 2, 1, -1, -2, -2, -1, 1, 2 };
int dy[] = { 1, 2, 2, 1, -1, -2, -2, -1 };

set<string>st;
bool ok ;
ll n,k;
void cnt_subseq_of_lenk(ll i,string &s, ll sz,string ans=""){
  if(sz(ans)==sz){
    st.emplace(ans);
    return;
  }
    if(i==sz(s)) return;
  if(sz(st)>=k){
    ok = 1;
    return;
  }
  cnt_subseq_of_lenk(i+1,s,sz,ans+s[i]);
  if(sz(st)>=k){
    ok = 1;
    return;
  }
  cnt_subseq_of_lenk(i+1,s,sz,ans);
}

bool comp(string &a,string &b){
    return sz(a) > sz(b);
}


void solve(){
 cin>>n>>k;
ll maxi = max(1LL,n-2);
long double l = 1LL<<maxi;
long double r = 1LL<<n;
if(r-l<k){
    cout<<-1<<nl;
    return;
}

string s; cin>>s;
for(int i=sz(s);i>=max(1LL,n-2);i--){
    if(ok) break;
    cnt_subseq_of_lenk(0,s,i);
}
if(sz(st)<k){
    cout<<-1<<nl;
    return;
}
ll res = 0;
vector<string>ans(all(st));
sort(all(ans),comp);
for(int i=0;i<sz(ans);i++){
    res += sz(s)-sz(ans[i]);
}
cout<<res<<nl;


}

int main(){
    Fast_IO();
int t =1;
//cin>>t;
while(t--){
solve();
}
return 0;
}