#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
const bool typetest = 0;
const int N = 4e2 + 5;
const int Inf = 1e9 + 7;
int n;
string s;
int dp[2][N][N][3];
int r[N], g[N], y[N];
int a, b, c;
void Read()
{
cin >> n >> s;
s = " " + s;
for (int i = 1; i <= n; ++i)
{
if (s[i] == 'R')
r[++a] = i;
else if (s[i] == 'G')
g[++b] = i;
else
y[++c] = i;
}
}
void Solve()
{
fill_n(&dp[0][0][0][0], 2 * N * N * 2, Inf);
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (int i = 0; i <= a; ++i)
{
for (int j = 0; j <= b; ++j)
for (int t = 0; t <= c; ++t)
{
if (i != 0)
dp[i & 1][j][t][0] = min(dp[!(i & 1)][j][t][1], dp[!(i & 1)][j][t][2]) + abs(i + j + t - r[i]);
else
dp[i & 1][j][t][0] = Inf * (i != 0 || j != 0 || t != 0);
if (j != 0)
dp[i & 1][j][t][1] = min(dp[i & 1][j - 1][t][0], dp[i & 1][j - 1][t][2]) + abs(i + j + t - g[j]);
else
dp[i & 1][j][t][1] = Inf * (i != 0 || j != 0 || t != 0);
if (t != 0)
dp[i & 1][j][t][2] = min(dp[i & 1][j][t - 1][0], dp[i & 1][j][t - 1][1]) + abs(i + j + t - y[t]);
else
dp[i & 1][j][t][2] = Inf * (i != 0 || j != 0 || t != 0);
}
}
int ans = min(dp[a & 1][b][c][0], min(dp[a & 1][b][c][1], dp[a & 1][b][c][2]));
cout << (ans == Inf ? -1 : ans / 2);
}
int32_t main()
{
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t(1);
if (typetest)
cin >> t;
for (int _ = 1; _ <= t; ++_)
{
Read();
Solve();
}
}
