#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9 + 7;
const int N = 405;
int pre[N][3];
inline void ckmin(int &x , int y){
x = min(x,y);
}
int eval(int tar , int c0 , int c1 , int c2){
if(tar == 0)return 0;
int temptar = tar;
tar -= min(c0 , pre[temptar-1][0]);
tar -= min(c1 , pre[temptar-1][1]);
tar -= min(c2 , pre[temptar-1][2]);
return tar;
}
void solve(){
int n;
cin >> n;
string str;
cin >> str;
vector < int > ind[3];
for(int i = 0;i<n;i++){
if(str[i] == 'R'){
ind[0].push_back(i);
pre[i][0] = 1;
}
else if(str[i] == 'G'){
ind[1].push_back(i);
pre[i][1] = 1;
}
else if(str[i] == 'Y'){
ind[2].push_back(i);
pre[i][2] = 1;
}
else assert(false);
}
for(int i = 0;i<3;i++){
for(int j = 1;j<n;j++){
pre[j][i] += pre[j-1][i];
}
}
int dp[ind[0].size()+2][ind[1].size()+2][ind[2].size()+2][3];
for(int i = 0;i<=ind[0].size();i++){
for(int j = 0;j<=ind[1].size();j++){
for(int k = 0;k<=ind[2].size();k++){
dp[i][j][k][0] = dp[i][j][k][1] = dp[i][j][k][2] = inf;
}
}
}
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for(int cnt0 = 0;cnt0 <= (int)ind[0].size();cnt0++){
for(int cnt1 = 0;cnt1 <= (int)ind[1].size();cnt1++){
for(int cnt2 = 0;cnt2 <= (int)ind[2].size();cnt2++){
if(cnt0 != (int)ind[0].size()){
ckmin(dp[cnt0+1][cnt1][cnt2][0] , dp[cnt0][cnt1][cnt2][1] + eval(ind[0][cnt0] , cnt0,cnt1,cnt2));
ckmin(dp[cnt0+1][cnt1][cnt2][0] , dp[cnt0][cnt1][cnt2][2] + eval(ind[0][cnt0] , cnt0,cnt1,cnt2));
}
if(cnt1 != (int)ind[1].size()){
ckmin(dp[cnt0][cnt1+1][cnt2][1] , dp[cnt0][cnt1][cnt2][0] + eval(ind[1][cnt1] , cnt0,cnt1,cnt2));
ckmin(dp[cnt0][cnt1+1][cnt2][1] , dp[cnt0][cnt1][cnt2][2] + eval(ind[1][cnt1] , cnt0,cnt1,cnt2));
}
if(cnt2 != (int)ind[2].size()){
ckmin(dp[cnt0][cnt1][cnt2+1][2] , dp[cnt0][cnt1][cnt2][0] + eval(ind[2][cnt2] , cnt0,cnt1,cnt2));
ckmin(dp[cnt0][cnt1][cnt2+1][2] , dp[cnt0][cnt1][cnt2][1] + eval(ind[2][cnt2] , cnt0,cnt1,cnt2));
}
}
}
}
int ans = inf;
for(int i = 0;i<3;i++){
ans = min(ans , dp[ind[0].size()][ind[1].size()][ind[2].size()][i]);
}
cout << (ans == inf ? -1 : ans) << endl;
}
signed main(){
ios_base::sync_with_stdio(0);cin.tie(0);
int testcase = 1;//cin >> testcase;
while(testcase--)solve();
}
