#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define INF 100000000
//#define int long long
#define lchild(i) (i*2 + 1)
#define rchild(i) (i*2 + 2)
#define mid(l, u) ((l+u)/2)
#define x(p) p.first
#define y(p) p.second
#define MOD 998244353
#define ordered_set tree<pair<int, int>, null_type,less<pair<int, int>>, rb_tree_tag,tree_order_statistics_node_update>
using namespace std;
int n;
const int N = 405;
int dp[N][N][N][4];
int arr[N];
vector<int> g[3];
int cmp[N][3];
int getdp(int i, int aused, int bused, int prev){
//cout<<i<<" "<<aused<<" "<<bused<<" "<<prev<<endl;
if(i>=n) return 0;
return dp[i][aused][bused][prev];
}
int getans(int i, int aused, int bused, int prev){
int cnt[] = {aused, bused, i - (aused + bused)};
int tr = INF;
for(int j = 0;j<3;j++){
if(j==prev || g[j].size() <= cnt[j]) continue;
int currind = g[j][cnt[j]];
for(int k = 0;k<3;k++){
if(k==j) continue;
currind += max((int)0, cnt[k] - cmp[g[j][cnt[j]]][k]);
//cout<<"equivalent at index "<<g[j][cnt[j]]<<" for type "<<k<<" is "<<cmp[g[j][cnt[j]]][k]<<endl;
}
cnt[j]++;
//cout<<"putting "<<j<<", cost is "<<(currind - (i+1))<<endl;
tr = min(tr, (currind - (i+1)) + getdp(i+1, cnt[0], cnt[1], j));
cnt[j]--;
}
return tr;
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
cin>>n;
string s;
cin>>s;
for(int i = 1;i<=n;i++){
if(s[i-1]=='R') arr[i] = 0;
else if(s[i-1]=='G') arr[i] = 1;
else arr[i] = 2;
cmp[i][0] = g[0].size();
cmp[i][1] = g[1].size();
cmp[i][2] = g[2].size();
g[arr[i]].push_back(i);
}
for(int i = 0;i<=n;i++) for(int j = 0;j<=n;j++) for(int k = 0;k<=n;k++) for(int x = 0;x<3;x++) dp[i][j][k][x] = INF;
for(int i = n;i>=1;i--){
for(int aused = n;aused>=0;aused--){
for(int bused = n;bused >= 0 ;bused--){
int cnt[] = {aused, bused, i - (aused + bused)};
if(cnt[0]>g[0].size() || cnt[1]>g[1].size() || cnt[2]>g[2].size()) continue;
if(cnt[2]<0) continue;
for(int prev = 0;prev<3;prev++){
int tr = INF;
for(int j = 0;j<3;j++){
if(j==prev || g[j].size() <= cnt[j]) continue;
int currind = g[j][cnt[j]];
for(int k = 0;k<3;k++){
if(k==j) continue;
currind += max((int)0, cnt[k] - cmp[g[j][cnt[j]]][k]);
//cout<<"equivalent at index "<<g[j][cnt[j]]<<" for type "<<k<<" is "<<cmp[g[j][cnt[j]]][k]<<endl;
}
cnt[j]++;
//cout<<"putting "<<j<<", cost is "<<(currind - (i+1))<<endl;
tr = min(tr, (currind - (i+1)) + getdp(i+1, cnt[0], cnt[1], j));
cnt[j]--;
}
dp[i][aused][bused][prev] = tr;
}
}
}
}
int ans = getans(0, 0, 0, 4);
if(ans>=INF) ans = -1;
cout<<ans<<"\n";
}
//2^(2k - 1) - 2^(k-1)
/*
4 4
RGWR
GRRG
WGGW
WWWR
5 5
RGRGW
GRRGW
WGGWR
RWRGW
RGWGW
20
YYGYYYGGGGRGYYGRGRYG
*/
