#include <bits/stdc++.h>
#define int long long
#define IO ios::sync_with_stdio(0), cin.tie(0)
#define FOR(i, a, b) for (int i = a, I = b; i <= I; i++)
using namespace std;
using pii = pair<int, int>;
void dbg() {;}
template<class T, class ...U>
void dbg(T a, U ...b) { cout << a << " "; dbg(b...); }
void ent() { cout << "\n"; }
const int INF = 1e16;
const int N = 405;
int r[N], g[N], y[N];
signed main() {
IO;
int n;
cin >> n;
string s;
cin >> s;
vector<int> rid, gid, yid;
rid.push_back(0);
gid.push_back(0);
yid.push_back(0);
FOR(i, 0, n - 1) {
if (s[i] == 'R')
r[i] = 1;
else if (s[i] == 'G')
g[i] = 1;
else
y[i] = 1;
if (s[i] == 'R')
rid.push_back(i);
else if (s[i] == 'G')
gid.push_back(i);
else
yid.push_back(i);
if (i) {
r[i] += r[i - 1];
g[i] += g[i - 1];
y[i] += y[i - 1];
}
}
int numr = rid.size() - 1, numg = gid.size() - 1, numy = yid.size() - 1;
// FOR(i, 0, n - 1) dbg(r[i]); ent();
// FOR(i, 0, n - 1) dbg(g[i]); ent();
// FOR(i, 0, n - 1) dbg(y[i]); ent();
vector<vector<int>> dpr(numr + 1, vector<int>(numg + 1, INF));
vector<vector<int>> dpg(numr + 1, vector<int>(numg + 1, INF));
vector<vector<int>> dpy(numr + 1, vector<int>(numg + 1, INF));
dpr[0][0] = dpg[0][0] = dpy[0][0] = 0;
for (int tot = 1; tot <= n; tot++) {
vector<vector<int>> ndpr(numr + 1, vector<int>(numg + 1, INF));
vector<vector<int>> ndpg(numr + 1, vector<int>(numg + 1, INF));
vector<vector<int>> ndpy(numr + 1, vector<int>(numg + 1, INF));
for (int i = 0; i <= numr; i++) {
for (int j = 0; i + j <= tot && j <= numg; j++) {
int k = tot - i - j;
if (k > numy) continue;
if (i)
ndpr[i][j] = min(
dpg[i - 1][j] + max(0ll, g[gid[j]] - g[rid[i]]) + max(0ll, y[yid[k]] - y[rid[i]]),
dpy[i - 1][j] + max(0ll, g[gid[j]] - g[rid[i]]) + max(0ll, y[yid[k]] - y[rid[i]])
);
if (j)
ndpg[i][j] = min(
dpr[i][j - 1] + max(0ll, r[rid[i]] - r[gid[j]]) + max(0ll, y[yid[k]] - y[gid[j]]),
dpy[i][j - 1] + max(0ll, r[rid[i]] - r[gid[j]]) + max(0ll, y[yid[k]] - y[gid[j]])
);
if (k)
ndpy[i][j] = min(
dpr[i][j] + max(0ll, r[rid[i]] - r[yid[k]]) + max(0ll, g[gid[j]] - g[yid[k]]),
dpg[i][j] + max(0ll, r[rid[i]] - r[yid[k]]) + max(0ll, g[gid[j]] - g[yid[k]])
);
}
}
swap(dpr, ndpr);
swap(dpg, ndpg);
swap(dpy, ndpy);
}
int ans = min({dpr[numr][numg], dpg[numr][numg], dpy[numr][numg]});
if (ans >= INF)
cout << -1 << "\n";
else
cout << ans << "\n";
return 0;
}
