#include <bits/stdc++.h>
using namespace std;
#define TASK "long"
#define fi first
#define se second
#define ll long long
#define pb push_back
#define ALL(x) (x).begin(), (x).end()
#define GETBIT(mask, i) ((mask) >> (i) & 1)
#define MASK(i) ((1LL) << (i))
#define SZ(x) ((int)(x).size())
#define mp make_pair
#define CNTBIT(mask) __builtin_popcount(mask)
template<class X, class Y> bool maximize(X &x, Y y){ if (x < y) {x = y; return true;} return false;};
template<class X, class Y> bool minimize(X &x, Y y){ if (x > y) {x = y; return true;} return false;};
typedef pair<int, int> ii;
const int N = 400 + 2;
const int inf = 1e9 + 7;
const long long INF = (long long)1e18 + 7;
const int mod = 1e9 + 7;
int n;
int a[N];
string s;
void read() {
cin >> n >> s;
for(int i = 0; i < n; i++) {
if (s[i] == 'R') a[i + 1] = 0;
else if (s[i] == 'G') a[i + 1] = 1;
else a[i + 1] = 2;
}
}
void solve() {
vector<int> pos[3];
for(int i = 1; i <= n; i++) pos[a[i]].push_back(i);
vector<vector<vector<int>>> dp[3];
for(int i = 0; i < 3; i++) {
dp[i].assign(SZ(pos[0]) + 1, vector<vector<int>>(SZ(pos[1]) + 1, vector<int>(SZ(pos[2]) + 1, inf)));
}
vector<tuple<int, int, int>> upd[n + 1];
for(int i = 0; i <= SZ(pos[0]); i++) {
for(int j = 0; j <= SZ(pos[1]); j++) {
for(int z = 0; z <= SZ(pos[2]); z++) {
// cerr << i + j + z << ' ' << i << ' ' << j << ' ' << z << endl;
upd[i + j + z].push_back({i, j, z});
}
}
}
for(int i = 0; i < 2; i++) dp[i][0][0][0] = 0;
auto countLess = [&](tuple<int, int, int> sz, int p) -> int {
// return 0;
int cnt = 0;
int s[] = {get<0>(sz), get<1>(sz), get<2>(sz)};
for(int i = 0; i < 3; i++) {
int t = lower_bound(pos[i].begin(), pos[i].begin() + s[i], p) - pos[i].begin();
cnt += s[i] - t;
}
// if (p == 1) cout << cnt << endl;
// if (p == 1) cout << cnt << endl;
return cnt;
};
for(int i = 0; i < n; i++) {
for(auto j: upd[i]) {
int c0 = get<0>(j);
int c1 = get<1>(j);
int c2 = get<2>(j);
for(int last = 0; last < 3; last++) {
if (dp[last][c0][c1][c2] < inf) {
// cerr << c0 << ' ' << c1 << ' ' << c2 << ' ' << last << ' ' << dp[last][c0][c1][c2] << endl;
tuple<int, int, int> sz = {c0, c1, c2};
int tmp = dp[last][c0][c1][c2];
if (last != 0 && c0 < SZ(pos[0])) {
int p = pos[0][c0];
int less = countLess(sz, p);
int cost = p + less - (i + 1);
minimize(dp[0][c0 + 1][c1][c2], tmp + cost);
}
if (last != 1 && c1 < SZ(pos[1])) {
int p = pos[1][c1];
int less = countLess(sz, p);
int cost = p + less - (i + 1);
minimize(dp[1][c0][c1 + 1][c2], tmp + cost);
}
if (last != 2 && c2 < SZ(pos[2])) {
int p = pos[2][c2];
int less = countLess(sz, p);
int cost = p + less - (i + 1);
minimize(dp[2][c0][c1][c2 + 1], tmp + cost);
}
}
}
}
}
int ans = inf;
for(int i = 0; i < 3; i++) ans = min(ans, dp[i][SZ(pos[0])][SZ(pos[1])][SZ(pos[2])]);
cout << (ans == inf ? -1 : ans);
}
signed main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
if (fopen(TASK".inp", "r")) {
freopen(TASK".inp", "r", stdin);
freopen(TASK".out", "w", stdout);
}
int test = 1;
// cin >> test;
while(test--) {
read();
solve();
}
return 0;
}
// rmq - rmq2d
// hash
// fw - fw2d
// segtree
