#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef long long ll;
typedef long double ld;
#define pb push_back
#define pii pair<int, int>
#define pll pair<ll, ll>
#define st first
#define nd second
#define ordered_set tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>
#define debug false
string s;
const int MAXN = 400 + 17;
vector<int> v;
int dp[MAXN][MAXN][MAXN][3];
int spref[MAXN][3];
int gdzie[MAXN][3];
int cnt[3];
int main () {
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n;
cin >> n >> s;
s = "#" + s;
v.pb(-1);
memset(gdzie[0], -1, sizeof(gdzie[0]));
memset(gdzie[1], -1, sizeof(gdzie[1]));
memset(gdzie[2], -1, sizeof(gdzie[2]));
for (int i = 1; i <= n; i ++) {
if (s[i] == 'R') {
v.pb(0);
} else if (s[i] == 'G') {
v.pb(1);
} else {
v.pb(2);
}
for (int k = 0; k < 3; k ++) {
spref[i][k] = spref[i - 1][k];
}
spref[i][v[i]] ++;
cnt[v[i]] ++;
gdzie[cnt[v[i]]][v[i]] = i;
//cout << cnt[v[i]] << " " << v[i] << ": " << i << "\n";
}
for (int i = 1; i <= n; i ++) {
for (int j = 0; j <= i; j ++) {
for (int k = 0; k <= i - j; k ++) {
for (int l = 0; l < 3; l ++) {
dp[i][j][k][l] = INT_MAX;
}
}
}
}
for (int i = 0; i <= n - 1; i ++) {
for (int j = 0; j <= i; j ++) {
for (int k = 0; k <= i - j; k ++) {
int l = i - j - k;
//dp[i + 1][j + 1][k][0] = min(dp[i][j][k][1])
//DODAJ 0
int x = gdzie[j + 1][0];
if (x != -1 && min(dp[i][j][k][1], dp[i][j][k][2]) != INT_MAX) {
x += max(0, k - spref[x][1]) + max(0, l - spref[x][2]);
dp[i + 1][j + 1][k][0] = min(dp[i + 1][j + 1][k][0], min(dp[i][j][k][1], dp[i][j][k][2]) + max(0, x - (i + 1)));
}
//DODAJ 1
x = gdzie[k + 1][1];
if (x != -1 && min(dp[i][j][k][0], dp[i][j][k][2]) != INT_MAX) {
x += max(0, j - spref[x][0]) + max(0, l - spref[x][2]);
dp[i + 1][j][k + 1][1] = min(dp[i + 1][j][k + 1][1], min(dp[i][j][k][0], dp[i][j][k][2]) + max(0, x - (i + 1)));
}
//DODAJ 2
x = gdzie[l + 1][2];
if (x != -1 && min(dp[i][j][k][0], dp[i][j][k][1]) != INT_MAX) {
x += max(0, j - spref[x][0]) + max(0, k - spref[x][1]);
dp[i + 1][j][k][2] = min(dp[i + 1][j][k][2], min(dp[i][j][k][0], dp[i][j][k][1]) + max(0, x - (i + 1)));
}
}
}
}
for (int i = 1; i <= n; i ++) {
for (int j = 0; j <= i; j ++) {
for (int k = 0; k <= i - j; k ++) {
for (int l = 0; l < 3; l ++) {
//cout << i << " " << j << " " << k << " " << l << ": " << dp[i][j][k][l] << "\n";
}
}
}
}
int minn = INT_MAX;
for (int l = 0; l < 3; l ++) {
minn = min(minn, dp[n][spref[n][0]][spref[n][1]][l]);
}
if (minn == INT_MAX) {
cout << -1 << "\n";
} else {
cout << minn << "\n";
}
return 0;
}
