#include <vector>
#include <iostream>
#include <algorithm>
#define int long long
using namespace std;
struct prefsum
{
int n;
vector<int> vals;
void init(vector<int> ip)
{
n = ip.size();
vals.resize(n + 1);
for (int i = 1; i <= n; i++)
{
vals[i] = vals[i - 1] + ip[i - 1];
}
}
int operator()(int l, int r)
{
return vals[r] - vals[l];
}
};
struct dpstruct
{
prefsum veg_R;
prefsum veg_Y;
prefsum veg_G;
vector<int> pos_R;
vector<int> pos_Y;
vector<int> pos_G;
int R = 0, Y = 0, G = 0;
vector<vector<vector<vector<int>>>> table;
int dp()
{
for (int r = 0; r <= R; r++)
{
for (int y = 0; y <= Y; y++)
{
for (int g = 0; g <= G; g++)
{
for (int last = 0; last < 3; last++)
{
int cost = 1e15;
if (last == 0 && r != 0)
{
cost = min(table[r - 1][y][g][1], table[r - 1][y][g][2]) + max(0LL, veg_Y(0, pos_R[r - 1]) - y) + max(0LL, veg_G(0, pos_R[r - 1]) - g);
}
else if (last == 1 && y != 0)
{
cost = min(table[r][y - 1][g][0], table[r][y - 1][g][2]) + max(0LL, veg_R(0, pos_Y[y - 1]) - r) + max(0LL, veg_G(0, pos_Y[y - 1]) - g);
}
else if (last == 2 && g != 0)
{
cost = min(table[r][y][g - 1][0], table[r][y][g - 1][1]) + max(0LL, veg_Y(0, pos_G[g - 1]) - y) + max(0LL, veg_R(0, pos_G[g - 1]) - r);
}
if (r + y + g != 0) table[r][y][g][last] = cost;
}
}
}
}
return min(table[R][Y][G][0], min(table[R][Y][G][1], table[R][Y][G][2]));
}
void init(int n, string s)
{
vector<int> rs(n), ys(n), gs(n);
for (int i = 0; i < n; i++)
{
char c = s[i];
if (c == 'R')
{
R++;
rs[i]++;
pos_R.push_back(i);
}
else if (c == 'Y')
{
Y++;
ys[i]++;
pos_Y.push_back(i);
}
else
{
G++;
gs[i]++;
pos_G.push_back(i);
}
}
veg_R.init(rs);
veg_Y.init(ys);
veg_G.init(gs);
table.resize(R + 1, vector<vector<vector<int>>>(Y + 1, vector<vector<int>>(G + 1, vector<int>(3, 0))));
}
};
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int n;
cin >> n;
string s;
cin >> s;
dpstruct dps;
dps.init(n, s);
int res = dps.dp();
if (res >= 1e15) cout << "-1\n";
else cout << res << "\n";
}
