#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
using vi = vector<int>;
using vvi = vector<vi>;
const int INF = 1'000'000;
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int N;
cin >> N;
string s;
cin >> s;
s = " " + s;
vi lst[3];
vvi ct(1+N, vi(3, 0));
for(int u = 0; u <= 2; u++)
lst[u].push_back(0);
vi S(1+N);
for(int i = 1; i <= N; i++)
{
if(s[i] == 'R')
S[i] = 0;
else if(s[i] == 'G')
S[i] = 1;
else
S[i] = 2;
ct[i] = ct[i-1];
ct[i][S[i]]++;
lst[S[i]].push_back(i);
}
int DP[1 + ct[N][0]][1 + ct[N][1]][1 + ct[N][2]][3];
for(int a = 0; a <= ct[N][0]; a++)
for(int b = 0; b <= ct[N][1]; b++)
for(int c = 0; c <= ct[N][2]; c++)
for(int x = 0; x <= 2; x++)
DP[a][b][c][x] = INF;
for(int x = 0; x <= 2; x++)
DP[0][0][0][x] = 0;
for(int a = 0; a <= ct[N][0]; a++)
{
for(int b = 0; b <= ct[N][1]; b++)
{
for(int c = 0; c <= ct[N][2]; c++)
{
for(int x = 0; x <= 2; x++)
{
int na = a, nb = b, nc = c;
if(x == 0 && a == 0)
continue;
if(x == 1 && b == 0)
continue;
if(x == 2 && c == 0)
continue;
int pos;
if(x == 0)
pos = lst[0][a];
else if(x == 1)
pos = lst[1][b];
else
pos = lst[2][c];
int delta = abs(ct[pos][0] - a) + abs(ct[pos][1] - b) + abs(ct[pos][2] - c);
if(x == 0)
na--;
else if(x == 1)
nb--;
else
nc--;
for(int y = 0; y <= 2; y++)
{
if(y == x) continue;
DP[a][b][c][x] = min(DP[a][b][c][x], DP[na][nb][nc][y] + delta);
}
}
}
}
}
int res = INF;
for(int x = 0; x <= 2; x++)
res = min(res, DP[ct[N][0]][ct[N][1]][ct[N][2]][x]);
if(res >= N*N)
res = -1;
else
res /= 2;
cout << res << '\n';
}
