//Make CSP great again
//Vengeance
#include <bits/stdc++.h>
#define TASK "TESTCODE"
#define Log2(x) 31 - __builtin_clz(x)
using namespace std;
const int N = 200;
vector<int> pos[3];
int cnt[3], dp[N + 1][N + 1][N + 1][3], f[2 * N + 1][3];
int n;
string s;
int convert(char c)
{
if (c == 'R')
{
return 0;
}
if (c == 'G')
{
return 1;
}
if (c == 'Y')
{
return 2;
}
}
void read()
{
pos[0] = pos[1] = pos[2] = {0};
cin >> n >> s;
int i = 0;
for (char &c : s)
{
++i;
++cnt[convert(c)];
for (int j = 0; j < 3; ++ j)
{
f[i][j] = f[i - 1][j];
}
++f[i][convert(c)];
pos[convert(c)].push_back(i);
}
}
bool minimize(int &a, int b)
{
if (a > b)
{
a = b;
return true;
}
return false;
}
int dis(int i, int j, int k, int t)
{
if (t == 0)
{
return pos[0][i] - i - f[min(pos[0][i], pos[1][j])][1] - f[min(pos[0][i], pos[2][k])][2];
}
if (t == 1)
{
return pos[1][j] - j - f[min(pos[1][j], pos[0][i])][0] - f[min(pos[1][j], pos[2][k])][2];
}
if (t == 2)
{
return pos[2][k] - k - f[min(pos[2][k], pos[0][i])][0] - f[min(pos[2][k], pos[1][j])][1];
}
}
void solve()
{
if (max({cnt[0], cnt[1], cnt[2]}) > (n + 1)/2)
{
cout << -1;
return ;
}
for (int i = 0; i <= cnt[0]; ++ i)
{
for (int j = 0; j <= cnt[1]; ++ j)
{
for (int k = 0; k <= cnt[2]; ++ k)
{
for (int l = 0; l < 3; ++ l)
{
if (i == 0 && j == 0 && k == 0)
{
dp[i][j][k][l] = 0;
}
else
{
dp[i][j][k][l] = 1e9;
}
}
if (i > 0)
{
minimize(dp[i][j][k][0], min(dp[i - 1][j][k][1], dp[i - 1][j][k][2]) + dis(i, j, k, 0));
}
if (j > 0)
{
minimize(dp[i][j][k][1], min(dp[i][j - 1][k][0], dp[i][j - 1][k][2]) + dis(i, j, k, 1));
}
if (k > 0)
{
minimize(dp[i][j][k][2], min(dp[i][j][k - 1][0], dp[i][j][k - 1][1]) + dis(i, j, k, 2));
}
//cerr << i << ' ' << j << ' ' << k << ' ' << dp[i][j][k][0] << ' ' << dp[i][j][k][1] << ' ' << dp[i][j][k][2] << '\n';
}
}
}
int res = 1e9;
for (int i = 0; i < 3; ++ i)
{
res = min(res, dp[cnt[0]][cnt[1]][cnt[2]][i]);
}
cout << res;
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
if (fopen(TASK".INP", "r"))
{
freopen(TASK".INP", "r", stdin);
//freopen(TASK".OUT", "w", stdout);
}
int t = 1;
bool typetest = false;
if (typetest)
{
cin >> t;
}
for (int __ = 1; __ <= t; ++ __)
{
//cout << "Case " << __ << ": ";
read();
solve();
}
}
