#include <bits/stdc++.h>
using namespace std;
int inf = 1e8;
int n;
int a[405];
char sir[405];
int dp[401][401][401][4];
int pos[3][405],p0,p1,p2;
int sp[3][405];
int main()
{
cin >> n;
for (int i = 1; i <= n; i++)
cin >> sir[i];
for (int i = 1; i <= n; i++)
{
if (sir[i] == 'R')
{
a[i] = 0;
pos[0][++p0] = i;
}
else if (sir[i] == 'G')
{
a[i] = 1;
pos[1][++p1] = i;
}
else
{
a[i] = 2;
pos[2][++p2] = i;
}
}
for (int i = 1; i <= n; i++)
{
for (int j = 0; j < 3; j++)
sp[j][i] = sp[j][i - 1];
sp[a[i]][i]++;
}
for (int i = 0; i <= p0; i++)
for (int j = 0; j <= p1; j++)
for (int q = 0; q <= p2; q++)
for (int c = 0; c < 3; c++)
dp[i][j][q][c] = inf;
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
for (int i = 0; i <= p0; i++)
{
for (int j = 0; j <= p1; j++)
{
for (int q = 0; q <= p2; q++)
{
if (i < p0)
{
int pz = pos[0][i + 1];
dp[i + 1][j][q][0] = min(dp[i + 1][j][q][0],min(dp[i][j][q][1],dp[i][j][q][2]) + abs(j - sp[1][pz]) + abs(q - sp[2][pz]));
}
if (j < p1)
{
int pz = pos[1][j + 1];
dp[i][j + 1][q][1] = min(dp[i][j + 1][q][1],min(dp[i][j][q][0],dp[i][j][q][2]) + abs(i - sp[0][pz]) + abs(q - sp[2][pz]));
}
if (q < p2)
{
int pz = pos[2][q + 1];
dp[i][j][q + 1][2] = min(dp[i][j][q + 1][2],min(dp[i][j][q][0],dp[i][j][q][1]) + abs(i - sp[0][pz]) + abs(j - sp[1][pz]));
}
}
}
}
int ans = min(dp[p0][p1][p2][0],min(dp[p0][p1][p2][1],dp[p0][p1][p2][2]));
if (ans >= inf / 2)
cout << -1;
else
cout << ans / 2;
return 0;
}
///dp[i][j][q][c] bla bla
