#include<bits/stdc++.h>
#define ll long long
#define ull unsigned ll
#define f first
#define s second
#define pii pair<int,int>
#define pll pair<ll,ll>
#define pb push_back
#define epb emplace_back
using namespace std;
int pref[401][3];
int dp[401][401][401][3];
vector <int> vec[3];
int main(){
int n; cin >> n;
char c[n + 1];
for(int i = 1; i <= n;i ++) cin >> c[i];
for(int i = 1; i <= n; i++){
for(int j = 0; j < 3; j++) pref[i][j] = pref[i - 1][j];
if(c[i] == 'R')
pref[i][0]++, vec[0].pb(i);
if(c[i] == 'G')
pref[i][1]++, vec[1].pb(i);
if(c[i] == 'Y')
pref[i][2]++, vec[2].pb(i);
}
for(int i = 0; i <= n; i++){
for(int j = 0; j <= n; j++){
for(int x= 0; x <= n; x++){
for(int k = 0; k < 3; k++){
dp[i][j][x][k] = 1e9;
}
}
}
}
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for(int i = 0; i <= (int)vec[0].size(); i++){
for(int j = 0; j <= (int)vec[1].size(); j++){
for(int k = 0; k <= (int)vec[2].size(); k++){
if(i){
int x = vec[0][i - 1];
int v = max(pref[x][1] - j, 0);
int u = max(pref[x][2] - k, 0);
dp[i][j][k][0] = min(dp[i - 1][j][k][1], dp[i - 1][j][k][2]) + u + v;
}
if(j){
int x = vec[1][j - 1];
int v = max(pref[x][0] - i, 0);
int u = max(pref[x][2] - k, 0);
dp[i][j][k][1] = min(dp[i][j - 1][k][0], dp[i][j - 1][k][2]) + u + v;
}
if(k){
int x = vec[2][k - 1];
int v = max(pref[x][0] - i, 0);
int u = max(pref[x][1] - j, 0);
dp[i][j][k][2] = min(dp[i][j][k - 1][0], dp[i][j][k - 1][1]) + u + v;
}
}
}
}
int l1 = vec[0].size();
int l2 = vec[1].size();
int l3 = vec[2].size();
int x = min({dp[l1][l2][l3][0], dp[l1][l2][l3][1], dp[l1][l2][l3][2]});
if(x == 1e9){
cout << -1 << "\n";
}
else cout << x;
return 0;
}
