#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
template<class T> using o_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
int main() {
ll n;
cin >> n;
string s;
cin >> s;
ll r = 0, g = 0, y = 0;
o_set<ll> R;
o_set<ll> G;
o_set<ll> Y;
for(ll j = 0; j < n; j++){
char i = s[j];
if(i == 'R'){
r++;
R.insert(j);
}else if(i == 'G'){
g++;
G.insert(j);
}else if(i == 'Y'){
y++;
Y.insert(j);
}
}
if(max(max(r, g), y) > (n + 1)/2){
cout << -1 << endl;
return 0;
}
if(n == 1){
cout << 0 << endl;
return 0;
}
//ll dp[n + 5][n + 5][n + 5][3]; // {0, 1, 2} = {R, G, Y}
vector<vector<vector<vector<ll>>>> dp(n + 5LL, vector<vector<vector<ll>>>(n + 5LL, vector<vector<ll>>(n + 5LL, {(ll)1e17, (ll)1e17, (ll)1e17})));
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for(ll i = 1; i <= n; i++){
for(ll number_r = 0; number_r <= min(i, r); number_r++){
for(ll number_g = 0; number_g <= min(i - number_r, g); number_g++){
ll number_y = i - number_g - number_r;
if(number_y > y){
continue;
}
if(number_r > 0){
ll prev = min(dp[number_r - 1][number_g][number_y][1], dp[number_r - 1][number_g][number_y][2]);
ll t = *R.find_by_order(number_r - 1LL);
ll temp = t;
temp -= min((ll)(G.order_of_key(t)), number_g);
temp -= min((ll)(Y.order_of_key(t)), number_y);
temp -= number_r - 1;
dp[number_r][number_g][number_y][0] = min(prev + temp, dp[number_r][number_g][number_y][0]);
}
if(number_g > 0){
ll prev = min(dp[number_r][number_g - 1][number_y][0], dp[number_r][number_g - 1][number_y][2]);
ll t = *G.find_by_order(number_g - 1LL);
ll temp = t;
temp -= min((ll)(R.order_of_key(t)), number_r);
temp -= min((ll)(Y.order_of_key(t)), number_y);
temp -= number_g - 1;
dp[number_r][number_g][number_y][1] = min(prev + temp, dp[number_r][number_g][number_y][1]);
}
if(number_y > 0){
ll prev = min(dp[number_r][number_g][number_y - 1][0], dp[number_r][number_g][number_y - 1][1]);
ll t = *Y.find_by_order(number_y - 1LL);
ll temp = t;
temp -= min((ll)(R.order_of_key(t)), number_r);
temp -= min((ll)(G.order_of_key(t)), number_g);
temp -= number_y - 1;
dp[number_r][number_g][number_y][2] = min(prev + temp, dp[number_r][number_g][number_y][2]);
}
}
}
}
ll ans = 1e17;
for(auto i : dp[r][g][y]){
ans = min(i, ans);
}
cout << ans;
cout << endl;
}
