#include <iostream>
#include <vector>
#include <cstring>
#pragma GCC optimize("Ofast", "unroll-loops", "no-stack-protector")
#define IO_OP std::ios::sync_with_stdio(0); std::cin.tie(0);
#define F first
#define S second
#define V vector
#define PB push_back
#define MP make_pair
#define EB emplace_back
#define ALL(v) (v).begin(), (v).end()
#define debug(x) cerr << #x << " is " << x << endl
using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef V<int> vi;
const int INF = 1e9 + 7, N = 404;
int dp[N][N][N][3], a[N], cnt[N][3];
vi occ[3];
signed main()
{
IO_OP;
int n;
string s;
cin >> n >> s;
for(int i=0;i<n;i++) {
a[i+1] = (s[i] == 'R' ? 0 : (s[i] == 'G' ? 1 : 2));
occ[a[i+1]].PB(i+1);
}
for(int i=1;i<=n;i++) {
for(int j=0;j<3;j++) cnt[i][j] = cnt[i-1][j];
cnt[i][a[i]]++;
}
memset(dp, 0x3f, sizeof dp);
for(int i=0;i<=cnt[n][0];i++) {
for(int j=0;j<=cnt[n][1];j++) {
for(int k=0;k<=cnt[n][2];k++) {
if(i == 0 && j == 0 && k == 0) {
for(int l=0;l<3;l++)
dp[i][j][k][l] = 0;
} else {
for(int l=0;l<3;l++) {
int pos, jump, pre_dp;
if(l == 0) {
if(i == 0) continue;
pre_dp = min(dp[i-1][j][k][1], dp[i-1][j][k][2]);
pos = occ[l][i-1];
jump = 0;
jump += max(0, cnt[pos-1][0] - (i-1));
jump += max(0, cnt[pos-1][1] - j);
jump += max(0, cnt[pos-1][2] - k);
} else if(l == 1) {
if(j == 0) continue;
pre_dp = min(dp[i][j-1][k][0], dp[i][j-1][k][2]);
pos = occ[l][j-1];
jump = 0;
jump += max(0, cnt[pos-1][0] - i);
jump += max(0, cnt[pos-1][1] - (j-1));
jump += max(0, cnt[pos-1][2] - k);
} else {
if(k == 0) continue;
pre_dp = min(dp[i][j][k-1][0], dp[i][j][k-1][1]);
pos = occ[l][k-1];
jump = 0;
jump += max(0, cnt[pos-1][0] - i);
jump += max(0, cnt[pos-1][1] - j);
jump += max(0, cnt[pos-1][2] - (k-1));
}
dp[i][j][k][l] = min(dp[i][j][k][l], pre_dp + jump);
}
}
}
}
}
int ans = INF;
for(int i=0;i<3;i++)
ans = min(ans, dp[cnt[n][0]][cnt[n][1]][cnt[n][2]][i]);
if(ans > 1e8) ans = -1;
cout << ans << endl;
}
