#include <bits/stdc++.h>
using namespace std;
void setup()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
}
int n, f[401][401][401][3], pre[3][401];
vector<int> p[3];
string s;
int main()
{
setup();
cin >> n >> s;
s = '#' + s;
p[0] = {0};
p[1] = {0};
p[2] = {0};
for (int i = 1; i <= n; ++i)
{
if (s[i] == 'R')
{
p[0].push_back(i);
}
else if (s[i] == 'G')
{
p[1].push_back(i);
}
else
{
p[2].push_back(i);
}
pre[0][i] = p[0].size();
pre[1][i] = p[1].size();
pre[2][i] = p[2].size();
}
for (int i = 0; i < p[0].size(); ++i)
{
for (int j = 0; j < p[1].size(); ++j)
{
for (int k = 0; k < p[2].size(); ++k)
{
if (i + j + k == 0)
{
f[i][j][k][0] = f[i][j][k][1] = f[i][j][k][2] = 0;
continue;
}
f[i][j][k][0] = f[i][j][k][1] = f[i][j][k][2] = 2e9;
if (0 < i)
{
f[i][j][k][0] = min(f[i - 1][j][k][1], f[i - 1][j][k][2]) + p[0][i] - i - j - k;
f[i][j][k][0] += max(0, pre[1][p[1][j]] - pre[1][p[0][i]]);
f[i][j][k][0] += max(0, pre[2][p[2][k]] - pre[2][p[0][i]]);
}
if (0 < j)
{
f[i][j][k][1] = min(f[i][j - 1][k][0], f[i][j - 1][k][2]) + p[1][j] - i - j - k;
f[i][j][k][1] += max(0, pre[0][p[0][i]] - pre[0][p[1][j]]);
f[i][j][k][1] += max(0, pre[2][p[2][k]] - pre[2][p[1][j]]);
}
if (0 < k)
{
f[i][j][k][2] = min(f[i][j][k - 1][0], f[i][j][k - 1][1]) + p[2][k] - i - j - k;
f[i][j][k][2] += max(0, pre[0][p[0][i]] - pre[0][p[2][k]]);
f[i][j][k][2] += max(0, pre[1][p[1][j]] - pre[1][p[2][k]]);
}
if (i + j + k == n)
{
n = min({f[i][j][k][0], f[i][j][k][1], f[i][j][k][2]});
}
}
}
}
cout << (n > 1e9 ? -1 : n);
return 0;
}
