#include <bits/stdc++.h>
using namespace std;
using ll = long long;
//#define int ll
using P = pair<int, int>;
#define all(x) x.begin(), x.end()
#define rep(i, l, n) for (int i = l; i < (n); ++i)
#define sz(x) (int)x.size()
const char nl = '\n';
const int mod = 998244353;
const int inf = 1e9;
void mnu(int &a, int b) {
if (a>b || a == -1)a=b;
}
void solve() {
int n; cin >> n;
string s; cin >> s; s = '&'+s;
deque<int> g[3];
rep(i, 1, n+1) {
if (s[i] == 'R')g[0].push_back(i);
else if (s[i] == 'G')g[1].push_back(i);
else g[2].push_back(i);
}
vector<int> vis(n+1, 2);
if (sz(g[0]) > sz(g[2]))swap(g[0], g[2]);
if (sz(g[0]) > sz(g[1]))swap(g[0], g[1]);
if (sz(g[1]) > sz(g[2]))swap(g[1], g[2]);
int dp[n+1][3][201][201];
memset(dp, -1, sizeof dp);
rep(i, 0, 3)
for (auto j: g[i])s[j] = char(i+'0'), vis[j] = i;
//cout << s << nl;
rep(i, 0, 3)
if (sz(g[i])>0)dp[1][i][i==0][i==1] = g[i][0]-1;
int a = sz(g[0]), b = sz(g[1]);
rep(i, 1, n+1) {
//dp[i][s[i]-'0'][(vis[i]==0)][(vis[i]==1)] = 0;
rep(pre_last, 0, 3) {
rep(zero, 0, min(i, sz(g[0])+1))
rep(bir, 0, min(i, sz(g[1])+1)) {
if (bir+zero>=i)break;
if (dp[i-1][pre_last][zero][bir] ==-1)continue;
if (sz(g[2]) < i-1-zero-bir)continue;
rep(last, 0, 3)
if (last != pre_last) {
int idx = (last==0?zero+1:0)+(last==1?bir+1:0)+(last==2?i-zero-bir:0);
if (sz(g[last])<idx)continue;
//assert(zero+bir+idx==i);
mnu(dp[i][last][zero+(last==0)][bir+(last==1)], dp[i-1][pre_last][zero][bir]+abs(g[last][idx-1]-i));
}
}
}
}
int res = inf;
rep(i, 0, 3)
rep(zero, 0, a+1)
rep(bir, 0, b+1) {
//if (dp[n][i][zero][bir])cout << i << " " << zero << " " << bir << nl;
if (~dp[n][i][zero][bir])res = min(res, dp[n][i][zero][bir]);
}
cout << (res==inf?-3:res+1)/2 << nl;
}
int32_t main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
solve();
return 0;
}
