/* code by the cute PixelCat owo */
/* as cute as nacho neko (aka. my wife)! */
//#pragma GCC optimize("O4,unroll-loops,no-stack-protector")
#include <bits/stdc++.h>
#define int LL //__int128
#define double long double
using namespace std;
using LL = long long;
using uLL = unsigned long long;
using pii = pair<int, int>;
#define For(i, a, b) for (int i = a; i <= b; i++)
#define Fors(i, a, b, s) for (int i = a; i <= b; i += s)
#define Forr(i, a, b) for (int i = a; i >= b; i--)
#define F first
#define S second
#define L(id) (id * 2 + 1)
#define R(id) (id * 2 + 2)
#define LO(x) (x & (-x))
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define sz(x) ((int)x.size())
#define mkp make_pair
#define MOD (int)(1000000007)
#define INF (int)(1000000000000000ll) // 1e15
#define EPS (1e-6)
#ifdef NYAOWO
#include "library/debug.hpp"
inline void USACO(const string &s) {
cerr << "USACO: " << s << "\n";
}
#else
#define debug(...)
inline void timer() {}
inline void USACO(const string &s) {
freopen((s + ".in").c_str(), "r", stdin);
freopen((s + ".out").c_str(), "w", stdout);
}
#endif
inline void NYA() {
ios::sync_with_stdio(false);
cin.tie(0);
}
inline int gcd(int a, int b) {
return __gcd(a, b);
}
inline int lcm(int a, int b) {
return a / gcd(a, b) * b;
}
int fpow(int b, int p, const int &mod) {
int ans = 1;
while (p) {
if (p & 1) ans = ans * b % mod;
p /= 2;
b = b * b % mod;
}
return ans;
}
template <typename T>
inline void chmin(T &_a, const T &_b) {
if (_b < _a) _a = _b;
}
template <typename T>
inline void chmax(T &_a, const T &_b) {
if (_b > _a) _a = _b;
}
// mt19937_64 rng(
// chrono::steady_clock::now().time_since_epoch().count());
const int MAXN=410;
vector<int32_t> p[3];
int32_t dp[2][MAXN][MAXN][3];
int32_t main() {
NYA();
// nyaacho >/////<
// miku sama bless me >/////<
int n; cin>>n;
string s; cin>>s;
For(i,0,2) p[i].eb(-1);
int cnt[3]={0};
For(i,1,n){
int j;
if(s[i-1]=='R') j=0;
else if(s[i-1]=='G') j=1;
else j=2;
p[j].eb(i);
cnt[j]++;
}
// For(i,0,2) debug(i,cnt[i],p[i]);
auto cost=[&](int i,int j,int k,int c,int x){
int ans=0;
ans+=lower_bound(p[0].begin()+1+i,p[0].end(),p[c][x])
-p[0].begin()-i-1;
ans+=lower_bound(p[1].begin()+1+j,p[1].end(),p[c][x])
-p[1].begin()-j-1;
ans+=lower_bound(p[2].begin()+1+k,p[2].end(),p[c][x])
-p[2].begin()-k-1;
return ans;
};
For(i,0,cnt[0]) For(j,0,cnt[1]) For(k,0,cnt[2]) For(end,0,2){
dp[i&1][j][k][end]=0x3f3f3f3f;
if(i+j+k==0){
dp[i&1][j][k][end]=0;
continue;
}
int ii=i&1;
int i2=!(i&1);
if(end==0){
if(!i) continue;
dp[ii][j][k][end]=min(
dp[i2][j][k][1],dp[i2][j][k][2]
)+cost(i,j,k,0,i);
}else if(end==1){
if(!j) continue;
dp[ii][j][k][end]=min(
dp[ii][j-1][k][0],dp[ii][j-1][k][2]
)+cost(i,j,k,1,j);
}else if(end==2){
if(!k) continue;
dp[ii][j][k][end]=min(
dp[ii][j][k-1][0],dp[ii][j][k-1][1]
)+cost(i,j,k,2,k);
}
}
auto ans=0x3f3f3f3f;
For(e,0,2) chmin(ans,dp[cnt[0]&1][cnt[1]][cnt[2]][e]);
if(ans<0x3f3f3f3f) cout<<ans<<"\n";
else cout<<"-1\n";
// debug(cost(1,1,1,2,2));
return 0;
}
