//Never stop trying
#include "bits/stdc++.h"
using namespace std;
#define boost ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0)
typedef long long ll;
typedef string str;
typedef long double ld;
typedef pair<int, int> pi;
#define fi first
#define se second
typedef vector<int> vi;
typedef vector<pi> vpi;
#define pb push_back
#define eb emplace_back
#define sz(x) (int)x.size()
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
#define endl "\n"
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
const int MOD = 1e9 + 7; //998244353
const ll INF = 1e9;
const int MX = 400 + 10;
const int nx[4] = {0, 0, 1, -1}, ny[4] = {1, -1, 0, 0}; //right left down up
template<class T> using V = vector<T>;
template<class T> bool ckmin(T& a, const T& b) { return a > b ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }
ll cdiv(ll a, ll b) { return a / b + ((a ^ b) > 0 && a % b); } // divide a by b rounded up
//constexpr int log2(int x) { return 31 - __builtin_clz(x); } // floor(log2(x))
mt19937 rng(chrono::system_clock::now().time_since_epoch().count());
//mt19937_64 rng(chrono::system_clock::now().time_since_epoch().count());
ll random(ll a, ll b){
return a + rng() % (b - a + 1);
}
#ifndef LOCAL
#define cerr if(false) cerr
#endif
#define dbg(x) cerr << #x << " : " << x << endl;
#define dbgs(x,y) cerr << #x << " : " << x << " / " << #y << " : " << y << endl;
#define dbgv(v) cerr << #v << " : " << "[ "; for(auto it : v) cerr << it << ' '; cerr << ']' << endl;
#define here() cerr << "here" << endl;
void IO() {
#ifdef LOCAL
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
#endif
}
/////////////////////////ONLY CLEAN CODES ALLOWED/////////////////////////
int N,A=0,B=0,C=0;
vi pos[3];
V<V<V<vi>>>memo,v;
void allocate(){
/*memo.resize(A+1);
FOR(i,0,A+1){
memo[i].resize(B+1);
FOR(j,0,B+1){
memo[i][j].resize(C+1);
FOR(k,0,C+1){
memo[i][j][k].assign(4,-1);
}
}
}*/
v.resize(3);
FOR(i,0,3){
v[i].resize(N+1);
FOR(j,0,N+1){
v[i][j].resize(3);
FOR(k,0,3){
v[i][j][k].assign(N+1,0);
}
}
}
}
void precompute(){
vi vec={A,B,C};
FOR(i,0,3) FOR(j,1,vec[i]+1){
int x=pos[i][j];
FOR(k,0,3) if(k!=i){
int cnt=0; v[i][j][k][0]=0;
FOR(l,1,vec[k]+1){
if(pos[k][l]>x) cnt++;
v[i][j][k][l]=cnt;
}
}
}
}
/*int solve(int a, int b, int c, int l){
if(a==A && b==B && c==C) return 0;
int &ind=memo[a][b][c][l];
if(ind!=-1) return ind;
int ans=INF;
if(l!=0 && a<A){
ckmin(ans,solve(a+1,b,c,0)+v[0][a+1][1][b]+v[0][a+1][2][c]);
}
if(l!=1 && b<B){
ckmin(ans,solve(a,b+1,c,1)+v[1][b+1][0][a]+v[1][b+1][2][c]);
}
if(l!=2 && c<C){
ckmin(ans,solve(a,b,c+1,2)+v[2][c+1][0][a]+v[2][c+1][1][b]);
}
return ind=ans;
}*/
void solve2(){
int dp[A+1][B+1][C+1][3];
FOR(i,0,A+1) FOR(j,0,B+1) FOR(k,0,C+1) FOR(l,0,3) dp[i][j][k][l]=INF;
FOR(i,0,3) dp[0][0][0][i]=0;
FOR(a,0,A+1) FOR(b,0,B+1) FOR(c,0,C+1){
if(!(a+b+c)){
continue;
}
if(a) dp[a][b][c][0]=min(dp[a-1][b][c][1],dp[a-1][b][c][2])
+v[0][a][1][b]+v[0][a][2][c];
if(b) dp[a][b][c][1]=min(dp[a][b-1][c][0],dp[a][b-1][c][2])
+v[1][b][0][a]+v[1][b][2][c];
if(c) dp[a][b][c][2]=min(dp[a][b][c-1][0],dp[a][b][c-1][1])
+v[2][c][0][a]+v[2][c][1][b];
}
int ans=min(min(dp[A][B][C][0],dp[A][B][C][1]),dp[A][B][C][2]);
if(ans==INF) ans=-1;
cout << ans << endl;
}
int main() {
boost; IO();
cin>>N;
pos[0].eb(); pos[1].eb(); pos[2].eb();
FOR(i,0,N){
char c; cin>>c;
if(c=='G'){
A++;
pos[0].eb(); pos[0][A]=i;
}
else if(c=='R'){
B++;
pos[1].eb(); pos[1][B]=i;
}
else{
C++;
pos[2].eb(); pos[2][C]=i;
}
}
allocate();
precompute();
/*int ans=solve(0,0,0,3);
if(ans==INF) ans=-1;
cout << ans << endl;*/
solve2();
return 0;
}
//Change your approach
