#include <bits/stdc++.h>
#define L(i, j, k) for(int i = (j); i <= (k); i++)
#define R(i, j, k) for(int i = (j); i >= (k); i--)
#define ll long long
#define sz(a) ((int) a.size())
#define all(a) a.begin(), a.end()
#define vi vector<ll>
#define pb emplace_back
#define me(a, x) memset(a, x, sizeof(a))
#define fst first
#define snd second
#define ii pair<int, int>
using namespace std;
int n;
bool valid(string s){
for(int i=1;i<n;i++){
if(s[i]==s[i-1])return false;
}
return true;
}
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin>>n;
string s;cin>>s;
if(valid(s)){cout<<0<<endl;return 0;}
queue<string>q;
q.push(s);
int ans=1e9;
unordered_map<string,int>d,v;
d[s]=0;
v[s]=1;
while(sz(q)){
string u=q.front();q.pop();
if(d[u]==n*n)continue;
for(int i=1;i<n;i++){
string c=u;
swap(c[i],c[i-1]);
if(v.count(c))continue;
d[c]=d[u]+1;
if(valid(c)){
ans=min(ans,d[c]);
}
v[c]=1;
q.push(c);
}
}
if(ans==1e9)cout<<-1<<endl;
else cout<<ans<<endl;
}
