#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 cerr if(false) cerr
#define watch(x) cerr << (#x) << " is " << (x) << endl;
#define endl '\n'
#define ld long double
#define int long long
#define pii pair<int, int>
#define fi first
#define se second
#define sz(a) (int)(a).size()
#define all(x) (x).begin(), (x).end()
int n;
string s;
void no() {
cout << -1 << endl;
exit(0);
}
signed main() {
ios::sync_with_stdio(0);
cin.tie(0);
cin >> n >> s;
int ans = 0;
for(int i = 1; i < n; i++) {
if(s[i] == s[i-1]) {
int ind = -1;
for(int j = i+1; j < n; j++) {
if((s[j] != s[i])) {
ind = j;
break;
}
}
if(ind == -1) no();
for(int j = ind; j >= i+1; j--) {
swap(s[j], s[j-1]);
ans++;
}
}
}
cout << ans << endl;
// cout << s << endl;
}
/*
Did you read the bounds?
Did you make typos?
Are there edge cases (N=1?)
Are array sizes proper?
Integer overflow?
DS reset properly between test cases?
Is using long longs causing TLE?
Are you using floating points?
*/
