#include <bits/stdc++.h>
#define ll long long
#define ull unsigned ll
#define pb push_back
#define pa pair<int, int>
#define pall pair<ll, int>
#define fi first
#define se second
#define TASK "test"
#define Size(x) (int) x.size()
#define all(x) x.begin(), x.end()
using namespace std;
template<typename T1, typename T2> bool mini (T1 &a, T2 b) {if(a > b) a = b; else return 0; return 1;}
template<typename T1, typename T2> bool maxi (T1 &a, T2 b) {if(a < b) a = b; else return 0; return 1;}
const int MOD = 1e9 + 7;
const int LOG = 20;
const int maxn = 400 + 7;
const ll oo = 1e9 + 69;
int n, dp[maxn][maxn][maxn][3];
char a[maxn];
vector<int> pos[3];
int convert(char ch) {
if(ch == 'R') return 0;
if(ch == 'G') return 1;
return 2;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//freopen(TASK".inp", "r", stdin);
//freopen(TASK".out", "w", stdout);
cin>>n;
for(int i = 1; i <= n; i++) {
cin>>a[i];
pos[convert(a[i])].pb(i);
}
for(int i = 0; i <= n; i++) {
for(int red = 0; red <= Size(pos[0]); red++) {
for(int green = 0; green <= Size(pos[1]); green++) {
for(int cur = 0; cur < 3; cur++) dp[i][red][green][cur] = oo;
}
}
}
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for(int i = 0; i < n; i++) {
for(int red = 0; red <= Size(pos[0]); red++) {
for(int green = 0; green <= Size(pos[1]); green++) {
int yellow = i - red - green;
if(yellow < 0 || yellow > Size(pos[2])) continue;
for(int cur = 0; cur < 3; cur++) {
int cost;
if(cur != 0 && red < Size(pos[0])) {
cost = 0;
int p = pos[0][red];
for(int j = 0; j < red; j++) cost += (p < pos[0][j]);
for(int j = 0; j < green; j++) cost += (p < pos[1][j]);
for(int j = 0; j < yellow; j++) cost += (p < pos[2][j]);
mini(dp[i + 1][red + 1][green][0], dp[i][red][green][cur] + cost);
}
if(cur != 1 && green < Size(pos[1])) {
cost = 0;
int p = pos[1][green];
for(int j = 0; j < red; j++) cost += (p < pos[0][j]);
for(int j = 0; j < green; j++) cost += (p < pos[1][j]);
for(int j = 0; j < yellow; j++) cost += (p < pos[2][j]);
mini(dp[i + 1][red][green + 1][1], dp[i][red][green][cur] + cost);
}
if(cur != 2 && yellow < Size(pos[2])) {
cost = 0;
int p = pos[2][yellow];
for(int j = 0; j < red; j++) cost += (p < pos[0][j]);
for(int j = 0; j < green; j++) cost += (p < pos[1][j]);
for(int j = 0; j < yellow; j++) cost += (p < pos[2][j]);
mini(dp[i + 1][red][green][2], dp[i][red][green][cur] + cost);
}
}
}
}
}
int ans = oo;
for(int i = 0; i < 3; i++) mini(ans, dp[n][Size(pos[0])][Size(pos[1])][i]);
cout<<(ans == oo ? -1:ans);
return 0;
}
