/** Im the best because i work as hard as i possibly can **/
#pragma GCC optimize("O2")
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
#define all(x) (x).begin(),(x).end()
#define F first
#define S second
#define Mp make_pair
#define fast_io ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define file_io freopen("in.txt" , "r+" , stdin) ; freopen("out.txt" , "w+" , stdout);
#define endl "\n"
const int N = 4e2 + 10;
const ll mod = 1e9 + 7;
const ll mod2 = 998244353;
const ll inf = 5e8;
const int LOG = 22;
ll pw(ll a , ll b, ll M) { return (!b ? 1 : (b & 1 ? (a * pw(a * a % M, b / 2, M)) % M : pw(a * a % M, b / 2, M))); }
/// dp[tp][i][j][k] -> age
char C[N];
string s;
int n, cnt[3], nxt[3][N][N][N], dp[3][N][N][N];
vector < int > pos[3];
inline int Less(int type, int x, int used)
{
return lower_bound(pos[type].begin() + used, pos[type].end(), x) - pos[type].begin() - used;
}
int main()
{
scanf("%d", &n);
scanf("%s", C);
s = C; s = "#" + s;
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';
cnt[s[i] - '0'] ++;
pos[s[i] - '0'].push_back(i);
}
for(int i = 0; i <= cnt[0]; i ++)
{
for(int j = 0; j <= cnt[1]; j ++)
{
for(int k = 0; k <= cnt[2]; k ++)
{
if(i != cnt[0])
{
int id = pos[0][i];
nxt[0][i][j][k] = Less(1, id, j) + Less(2, id, k);
}
else nxt[0][i][j][k] = inf;
if(j != cnt[1])
{
int id = pos[1][j];
nxt[1][i][j][k] = Less(0, id, i) + Less(2, id, k);
}
else nxt[1][i][j][k] = inf;
if(k != cnt[2])
{
int id = pos[2][k];
nxt[2][i][j][k] = Less(0, id, i) + Less(1, id, j);
}
else nxt[2][i][j][k] = inf;
///printf("i = %d j = %d k = %d nxt[0] = %d nxt[1] = %d nxt[2] = %d\n", i, j, k, nxt[0][i][j][k], nxt[1][i][j][k], nxt[2][i][j][k]);
}
}
}
///printf("\n\n");
for(int i = 0; i < N; i ++) for(int j = 0; j < N; j ++) dp[0][0][i][j] = dp[1][0][i][j] = dp[2][0][i][j] = inf;
dp[0][0][0][0] = dp[1][0][0][0] = dp[2][0][0][0] = 0;
for(int i = 1; i <= n; i ++)
{
for(int j = 0; j <= min(i, cnt[0]); j ++)
{
for(int k = 0; k <= min(i - j, cnt[1]); k ++)
{
dp[0][i][j][k] = dp[1][i][j][k] = dp[2][i][j][k] = inf;
if(i - j - k > cnt[2])
{
continue;
}
if(j) dp[0][i][j][k] = min(dp[1][i - 1][j - 1][k], dp[2][i - 1][j - 1][k]) + nxt[0][j - 1][k][i - j - k];
if(k) dp[1][i][j][k] = min(dp[0][i - 1][j][k - 1], dp[2][i - 1][j][k - 1]) + nxt[1][j][k - 1][i - j - k];
if(i != j + k) dp[2][i][j][k] = min(dp[0][i - 1][j][k], dp[1][i - 1][j][k]) + nxt[2][j][k][i - j - k - 1];
}
}
}
/*for(int i = 1; i <= n; i ++)
{
for(int j = 0; j <= cnt[0]; j ++)
{
for(int k = 0; k <= cnt[1]; k ++)
{
printf("i = %d j = %d k = %d dp[0] = %d dp[1] = %d dp[2] = %d\n", i, j, k, dp[0][i][j][k], dp[1][i][j][k], dp[2][i][j][k]);
}
}
}
*/
int ans = min({dp[0][n][cnt[0]][cnt[1]], dp[1][n][cnt[0]][cnt[1]], dp[2][n][cnt[0]][cnt[1]]});
if(ans >= inf) ans = -1;
printf("%d\n", ans);
return 0;
}
