#pragma GCC optimize("Ofast","unroll-loops","omit-frame-pointer","inline") //Optimization flags
#pragma GCC option("arch=native","tune=native","no-zero-upper") //Enable AVX
#pragma GCC target("avx2") //Enable AVX
#include<bits/stdc++.h>
using namespace std;
#define all(a) begin(a),end(a)
#define F first
#define S second
#define pb push_back
#define mp make_pair
typedef long long ll;
typedef vector<int> vi;
typedef pair<int,int> pi;
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#endif
void clr(auto &a,int n){
a.clear();
a.resize(n);
}
template <typename A, typename B>
istream& operator>>(istream& input,pair<A,B>& x) {
input>>x.F>>x.S;
return input;
}
template <typename A>
istream& operator>>(istream& input,vector<A>& x) {
for(auto& i:x)
input>>i;
return input;
}
template<typename A>
ostream& operator<<(ostream& output,vector<A>& x) {
for(auto& i:x)
output<<i<<' ';
return output;
}
template<typename T>
vector<pair<T,int>> getvec(int n){
vector<pair<T,int>>a(n);
for(int i=0;i<a.size();i++){
cin>>a[i].F;
a[i].S=i;
}
return a;
}
const int mod=1e9+7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int mul(int a,int b){
return ((a)*1ll*(b))%mod;
}
void add(int &a,int b){
a+=b;
if(a>=mod)a-=mod;
}
int sub(int a,int b){
a-=b;
if(a<0){
a+=mod;
}
return a;
}
int powz(int a,int b){
int res=1;
while(b){
if(b&1){
res=mul(res,a);
}
b/=2;
a=mul(a,a);
}
return res;
}
const int N=410;
int dp[250][250][3],new_dp[250][250][3];
vector<int>adj[3];
struct ordered_set {
int sz = 0, bit[N];
int size() {
return sz;
}
void update(int k, int x) {
k++;
while (k < N) {
bit[k] += x;
k += k & -k;
}
sz += x;
}
int find_by_order(int k) {
int ans = 0, sum = 0;
for (int j = 17; j >= 0; --j) {
ans += 1 << j;
if (ans < N && sum + bit[ans] < k) {
sum += bit[ans];
} else {
ans -= 1 << j;
}
}
return ans + 1;
}
int order_of_key(int k) {
k++;
int ans = 0;
while (k >= 1) {
ans += bit[k];
k -= k & -k;
}
return ans - 1;
}
}st[250][250];
vector<int>rv;
void solve(){
int n;
cin>>n;
string s;
cin>>s;
rv.resize(n);
for(int i=0;i<s.length();i++){
if(s[i]=='R'){
adj[0].pb(i);
rv[i]=adj[0].size()-1;
}
else if(s[i]=='G'){
adj[1].pb(i);
rv[i]=adj[1].size()-1;
}
else{
adj[2].pb(i);
}
}
int mx=max({(int)adj[0].size(),(int)adj[1].size(),(int)adj[2].size()});
if(mx*2>(n+10)){
cout<<-1;
return;
}
for(int i=0;i<=adj[0].size();i++){
for(int j=0;j<=adj[1].size();j++){
for(int k=0;k<n;k++){
if(s[k]=='R'){
if(rv[k]>=i){
st[i][j].update(k,1);
}
}
else if(s[k]=='G'){
if(rv[k]>=j){
st[i][j].update(k,1);
}
}
else{
st[i][j].update(k,1);
}
}
}
}
for(int j=0;j<=adj[0].size();j++){
for(int k=0;k<=adj[1].size();k++){
for(int l=0;l<3;l++){
dp[j][k][l]=new_dp[j][k][l]=1e9;
}
}
}
dp[0][0][0]=0;
dp[0][0][1]=0;
dp[0][0][2]=0;
for(int i=0;i<n;i++){
for(int j=0;j<=min(i,(int)adj[0].size());j++){
for(int k=0;k<=min(i,(int)adj[1].size());k++){
if(i-(j+k)<0){
break;
}
//for(int l=0;l<3;l++){
int l=0;
if(dp[j][k][l]!=1e9){
int put=0;
if(put!=l){
if(j+1<=adj[0].size()){
int fnd=st[j][k].order_of_key(adj[0][j]);
new_dp[j+1][k][0]=min(new_dp[j+1][k][0],dp[j][k][l]+fnd);}
}
put=1;
if(put!=l){
if(k+1<=adj[1].size()){
int fnd=st[j][k].order_of_key(adj[1][k]);
new_dp[j][k+1][1]=min(new_dp[j][k+1][1],dp[j][k][l]+fnd);}
}
put=2;
if(put!=l){
if((i-(j+k)+1)<=adj[2].size()){
int fnd=st[j][k].order_of_key(adj[2][i-(j+k)]);
new_dp[j][k][2]=min(new_dp[j][k][2],dp[j][k][l]+fnd);}
}
}
l=1;
if(dp[j][k][l]!=1e9){
int put=0;
if(put!=l){
if(j+1<=adj[0].size()){
int fnd=st[j][k].order_of_key(adj[0][j]);
new_dp[j+1][k][0]=min(new_dp[j+1][k][0],dp[j][k][l]+fnd);}
}
put=1;
if(put!=l){
if(k+1<=adj[1].size()){
int fnd=st[j][k].order_of_key(adj[1][k]);
new_dp[j][k+1][1]=min(new_dp[j][k+1][1],dp[j][k][l]+fnd);}
}
put=2;
if(put!=l){
if((i-(j+k)+1)<=adj[2].size()){
int fnd=st[j][k].order_of_key(adj[2][i-(j+k)]);
new_dp[j][k][2]=min(new_dp[j][k][2],dp[j][k][l]+fnd);}
}
}
l=2;
if(dp[j][k][l]!=1e9){
int put=0;
if(put!=l){
if(j+1<=adj[0].size()){
int fnd=st[j][k].order_of_key(adj[0][j]);
new_dp[j+1][k][0]=min(new_dp[j+1][k][0],dp[j][k][l]+fnd);}
}
put=1;
if(put!=l){
if(k+1<=adj[1].size()){
int fnd=st[j][k].order_of_key(adj[1][k]);
new_dp[j][k+1][1]=min(new_dp[j][k+1][1],dp[j][k][l]+fnd);}
}
put=2;
if(put!=l){
if((i-(j+k)+1)<=adj[2].size()){
int fnd=st[j][k].order_of_key(adj[2][i-(j+k)]);
new_dp[j][k][2]=min(new_dp[j][k][2],dp[j][k][l]+fnd);}
}
}
}
}
for(int j=0;j<=min(i+1,(int)adj[0].size());j++){
for(int k=0;k<=min(i+1,(int)adj[1].size());k++){
if(i-(j+k)<0){
break;
}
if(i+1-(j+k)>adj[2].size()){
continue;
}
st[j][k].update(adj[2][i-(j+k)],-1);
}
}
for(int j=0;j<=(int)adj[0].size();j++){
for(int k=0;k<=(int)adj[1].size();k++){
for(int l=0;l<3;l++){
dp[j][k][l]=new_dp[j][k][l];
new_dp[j][k][l]=1e9;
}
}
}
}
int ans=1e9;
ans=min(ans,dp[adj[0].size()][adj[1].size()][0]);
ans=min(ans,dp[adj[0].size()][adj[1].size()][1]);
ans=min(ans,dp[adj[0].size()][adj[1].size()][2]);
if(ans==1e9){
ans=-1;
}
cout<<ans;
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int tc=1;
//cin>>tc;
for(int _=0;_<tc;_++){
// cout<<"Case #"<<_+1<<": ";
solve();
if(_!=tc-1){
cout<<'\n';
}
}
}
