#include <bits/stdc++.h>
using namespace std;
int dp[407][407][407][4];
int pom[3][407][407];
vector<int> pos[3];
int main()
{
ios::sync_with_stdio(0);
cin.tie(0);
int n;
string s;
cin >> n >> s;
for (int i = 0; i < n; i++)
{
if(s[i] == 'R')
{
pos[0].push_back(i);
}
else if(s[i] == 'G')
{
pos[1].push_back(i);
}
else if(s[i] == 'Y')
{
pos[2].push_back(i);
}
}
for (int i = 0; i < 3; i++)
{
if (pos[i].size() > (n + 1) / 2)
{
cout << -1 << endl;
return 0;
}
}
for (int i = 0; i < 3; i++)
{
for (int j = 0; j <= pos[i].size(); j++)
{
for (int k = 0; k <= n; k++)
{
if(j == 0)
{
pom[i][j][k] = 0;
}
else
{
pom[i][j][k] = pom[i][j - 1][k];
if(k > pos[i][j - 1])
{
pom[i][j][k]++;
}
}
}
}
}
for (int i = 0; i <= pos[0].size(); i++)
{
for (int j = 0; j <= pos[1].size(); j++)
{
for (int k = 0; k <= pos[2].size(); k++)
{
for (int l = 0; l < 4; l++)
{
dp[i][j][k][l] = 8000000;
}
}
}
}
dp[0][0][0][3] = 0;
for(int i = 0; i <= pos[0].size(); i++)
{
for(int j = 0; j <= pos[1].size(); j++)
{
for(int k = 0; k <= pos[2].size(); k++)
{
for(int pop = 0; pop < 4; pop++)
{
if(dp[i][j][k][pop] == 8000000)
{
continue;
}
for (int c = 0; c < 3; c++)
{
if(pop != 3 && c == pop)
{
continue;
}
if(c == 0 && i < pos[0].size())
{
dp[i + 1][j][k][0] = min(dp[i + 1][j][k][0], dp[i][j][k][pop] + pos[0][i] - (pom[0][i][pos[0][i]] + pom[1][j][pos[0][i]] + pom[2][k][pos[0][i]]));
}
if(c == 1 && j < pos[1].size())
{
dp[i][j + 1][k][1] = min(dp[i][j + 1][k][1], dp[i][j][k][pop] + pos[1][j] - (pom[0][i][pos[1][j]] + pom[1][j][pos[1][j]] + pom[2][k][pos[1][j]]));
}
if(c == 2 && k < pos[2].size())
{
dp[i][j][k + 1][2] = min(dp[i][j][k + 1][2], dp[i][j][k][pop] + pos[2][k] - (pom[0][i][pos[2][k]] + pom[1][j][pos[2][k]] + pom[2][k][pos[2][k]]));
}
}
}
}
}
}
int odp = 8000000;
for (int i = 0; i < 3; i++)
{
odp = min(odp, dp[pos[0].size()][pos[1].size()][pos[2].size()][i]);
}
if(odp == 8000000)
{
cout << -1 << endl;
}
else
{
cout << odp << endl;
}
return 0;
}
