#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define fi first
#define si second
typedef pair<int,int> pi;
typedef tuple<int,int,int> ti;
template<typename T> bool chmin(T &a, T b){return (b < a) ? a = b, 1 : 0;}
template<typename T> bool chmax(T &a, T b){return (b > a) ? a = b, 1 : 0;}
void debug_out() {cerr<<endl;}
template <typename Head, typename... Tail>
void debug_out(Head _H, Tail... _T) {cerr<<" "<<to_string(_H);debug_out(_T...);}
#define debug(...) cerr<<"["<<#__VA_ARGS__<<"]:",debug_out(__VA_ARGS__)
const int N = 410, INF=2e9;
int n, s[N];
int dp[N][N][N][3];
int pref[3][N], pos[3][N];
int rc,gc,yc;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
cin.exceptions(ios::badbit|ios::failbit);
cin >> n;
for (int i = 1; i <= n; ++i) {
char x; cin >> x;
if (x=='R') s[i]=0,++rc,pos[0][rc]=i;
else if (x=='G') s[i]=1,++gc,pos[1][gc]=i;
else s[i]=2,++yc,pos[2][yc]=i;
pref[0][i]=rc;
pref[1][i]=gc;
pref[2][i]=yc;
}
for (int i = 0; i <= n; ++i) {
for (int j = 0; j <= n; ++j) {
for (int k = 0; k <= n; ++k) {
fill(dp[i][j][k],dp[i][j][k]+3,INF);
}
}
}
for (int i = 0; i < 3; ++i) dp[0][0][0][i]=0;
for (int i = 0; i <= rc; ++i) {
for (int j = 0; j <= gc; ++j) {
for (int k = 0; k <= yc; ++k) {
if (i==0&&j==0&k==0) continue;
if (i) { // place red
int og = pos[0][i];
dp[i][j][k][0]=min(dp[i-1][j][k][1],dp[i-1][j][k][2]);
dp[i][j][k][0]+=max(0, pref[1][og]-j)+max(0, pref[2][og]-k);
}
if (j) {
int og = pos[1][j];
dp[i][j][k][1]=min(dp[i][j-1][k][0],dp[i][j-1][k][2]);
dp[i][j][k][1]+=max(0, pref[0][og]-i)+max(0, pref[2][og]-k);
}
if (k) {
int og = pos[2][k];
dp[i][j][k][2]=min(dp[i][j][k-1][0],dp[i][j][k-1][1]);
dp[i][j][k][2]+=max(0, pref[0][og]-i)+max(0, pref[1][og]-j);
}
}
}
}
int ans = INF;
for (int i = 0; i < 3; ++i) chmin(ans, dp[rc][gc][yc][i]);
cout << (ans==INF?-1:ans);
return 0;
}
