//Sylwia Sapkowska
#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
using namespace std;
void __print(int x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << "'" << x << "'";}
void __print(const char *x) {cerr << '"' << x << '"';}
void __print(const string &x) {cerr << '"' << x << '"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef LOCAL
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
#define int long long
typedef pair<int, int> T;
const int oo = 1e18, oo2 = 1e9+7, K = 30;
const int mod = 998244353;
int count(string &s, string t){
int ret = 0;
int n = (int)s.size();
for (int i = 0; i<n; i++){
if (s[i] == t[i]) continue;
for (int j = i+1; j<n; j++){
if (s[i] == t[j]){
ret += (j-i);
string r;
for (int rep = 0; rep < i; rep++) r += s[rep];
r += t[j];
for (int rep = i; rep < n; rep++) {
if (rep == j) continue;
r += t[rep];
}
swap(t, r);
break;
}
}
}
return ret;
}
void solve(){
int n; cin >> n;
string s; cin >> s;
int r = 0, g = 0, y = 0;
for (auto c: s){
if (c == 'R') r++;
if (c == 'G') g++;
if (c == 'Y') y++;
}
int ans = oo;
for (int mask = 0; mask < (1<<n); mask++){
for (int submask = mask; ; submask = (submask-1)&mask){
int R = __builtin_popcountll(submask);
int G = __builtin_popcountll(mask) - R;
int Y = n - __builtin_popcountll(mask);
if (R != r || G != g || Y != y) {
if (!submask) break;
continue;
}
string t;
for (int i = 0; i<n; i++){
if (submask&(1<<i)){
t += 'R';
} else if (mask&(1<<i)){
t += 'G';
} else {
t += 'Y';
}
}
bool ok = 1;
for (int i = 1; i<n; i++){
ok &= (t[i] != t[i-1]);
}
if (ok) ans = min(ans, count(s, t));
if (!submask) break;
}
}
if (ans == oo) cout << "-1\n";
else cout << ans << "\n";
}
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
//cin >> t;
while (t--) solve();
return 0;
}
