/*input
20
YYGYYYGGGGRGYYGRGRYG
*/
#pragma GCC optimize ("O3")
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
map<tuple<int, int, int, int>, int>M;
int dp(bitset<400>s[3], int x, int n)
{
int h1 = hash<bitset<400>>()(s[0]);
int h2 = hash<bitset<400>>()(s[1]);
int h3 = hash<bitset<400>>()(s[2]);
if (M.count(make_tuple(h1, h2, h3, x)))
return M[make_tuple(h1, h2, h3, x)];
if (n == 1)
{
if (s[x][0])
return 0;
else
return 3e8;
}
for (int i = n - 1; i >= 0; i--)
{
if (s[x][i])
{
bitset<400>ss[3] = {s[0], s[1], s[2]};
ss[x][i] = 0;
for (int j = i; j + 1 < n; j++)
{
ss[0][j] = ss[0][j + 1];
ss[1][j] = ss[1][j + 1];
ss[2][j] = ss[2][j + 1];
}
int cost = (n - 1) - i;
int mini = 3e8;
for (int y = 0; y < 3; y++)
{
if (x != y)
{
mini = min(mini, dp(ss, y, n - 1));
}
}
return M[make_tuple(h1, h2, h3, x)] = cost + mini;
}
}
return M[make_tuple(h1, h2, h3, x)] = 3e8;
}
int main()
{
ios_base::sync_with_stdio(false);
int n;
string s;
cin >> n >> s;
bitset<400>ss[3];
for (int i = 0; i < n; i++)
{
if (s[i] == 'R')
ss[0][i] = true;
if (s[i] == 'G')
ss[1][i] = true;
if (s[i] == 'Y')
ss[2][i] = true;
}
int ans = min({dp(ss, 0, n), dp(ss, 1, n), dp(ss, 2, n)});
if (ans > 1e8)
ans = -1;
cout << ans << "\n";
}
