#include <bits/stdc++.h>
using namespace std;
int n;
string s;
int moves;
void repair(int idx){
while(idx - 1 > 0 && s[idx] != s[idx-1] && s[idx-1] == s[idx-2]){
swap(s[idx], s[idx-1]);
moves++;
idx--;
}
}
int solve(){
for(int i = 1; i < n; i++)
if(s[i] != s[i-1]) repair(i);
reverse(s.begin(), s.end());
for(int i = 1; i < n; i++)
if(s[i] != s[i-1]) repair(i);
for(int i = 1; i < n; i++)
if(s[i] == s[i-1]) return - 1;
return moves;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n >> s;
cout << solve() << "\n";
}
