#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 };


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

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


void solve(){
ll n,k; cin>>n>>k;

long double x = 1LL<<n;
if(k>x){
  cout<<0<<nl;
  return;
}

string s; cin>>s;
cnt_subseq_of_lenk(0,s);
vector<string>v(all(st));
sort(all(v),comp);
ll ans = 0;
for(int i=0;i<sz(v);i++){
    if(!k)break;
    k--;
    ans+=sz(s)-sz(v[i]);
}
if(k) ans = -1;
cout<<ans<<nl;


}

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