#include <algorithm>
#include <iostream>
#include <numeric>
#include <cstring>
#include <iomanip>
#include <vector>
#include <bitset>
#include <stack>
#include <queue>
#include <cmath>
#include <set>
#include <map>
#define endl '\n'
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
const int maxn = 410;
const int inf = 1e8;
int dp[2][maxn][maxn][3];
int idxs[3][maxn];
int ps[maxn][3];
int cnt[3];
int32_t main()
{
ios::sync_with_stdio(false);
cin.tie(0);
int n; cin >> n;
string _s; cin >> _s;
vector<int> s;
for(int i = 0; i < n; i++)
{
char ch = _s[i];
if(ch == 'R')
s.push_back(0);
else if(ch == 'G')
s.push_back(1);
else
s.push_back(2);
idxs[s.back()][cnt[s.back()]++] = i;
for(int j = 0; j < 3; j++)
ps[i+1][j] = ps[i][j];
ps[i+1][s.back()]++;
}
if(2*max(cnt[0], max(cnt[1], cnt[2])) > n+1)
{
cout << -1 << endl;
return 0;
}
//for(int i = 0; i <= n; i++)
// //cerr << ps[i][0] << " " << ps[i][1] << " " << ps[i][2] << endl;
for(int x = 0; x < 2; x++)
for(int y = 0; y <= cnt[1]; y++)
for(int z = 0; z <= cnt[2]; z++)
for(int c = 0; c < 3; c++)
dp[x][y][z][c] = inf;
for(int x = 0; x <= cnt[0]; x++)
{
for(int y = 0; y <= cnt[1]; y++)
for(int z = 0; z <= cnt[2]; z++)
for(int c = 0; c < 3; c++)
dp[x&1][y][z][c] = inf;
for(int y = 0; y <= cnt[1]; y++)
{
for(int z = 0; z <= cnt[2]; z++)
{
if(x == 0 and y == 0 and z == 0)
{
dp[x&1][y][z][0] = 0;
dp[x&1][y][z][1] = 0;
dp[x&1][y][z][2] = 0;
continue;
}
if(x > 0)
{
int i = idxs[0][x-1];
int a = i - min(ps[i][0],x) - min(ps[i][1],y) - min(ps[i][2],z);
dp[x&1][y][z][0] = min(dp[!(x&1)][y][z][1], dp[!(x&1)][y][z][2]) + a;
////cerr << a << " ";
}
if(y > 0)
{
int i = idxs[1][y-1];
int a = i - min(ps[i][0],x) - min(ps[i][1],y) - min(ps[i][2],z);
dp[x&1][y][z][1] = min(dp[x&1][y-1][z][0], dp[x&1][y-1][z][2]) + a;
//cerr << a << " ";
}
if(z > 0)
{
int i = idxs[2][z-1];
int a = i - min(ps[i][0],x) - min(ps[i][1],y) - min(ps[i][2],z);
dp[x&1][y][z][2] = min(dp[x&1][y][z-1][0], dp[x&1][y][z-1][1]) + a;
//cerr << a << " ";
}
//cerr << endl;
//cerr << x << "," << y << "," << z << ": " << dp[x&1][y][z][0] << " " << dp[x&1][y][z][1] << " " << dp[x&1][y][z][2] << endl;
}
}
}
int ans = min(dp[cnt[0]&1][cnt[1]][cnt[2]][0], min(dp[cnt[0]&1][cnt[1]][cnt[2]][1], dp[cnt[0]&1][cnt[1]][cnt[2]][2]));
if(ans >= inf)
ans = -1;
cout << ans << endl;
return 0;
}
