#include <bits/stdc++.h>
using i64 = long long;
#define all(x) (x).begin(), (x).end()
const int N = 400 + 5;
const int INF = 1E9;
int n, pre[3][N][3];
std::vector<int> idxs[3];
std::vector<std::vector<std::vector<std::vector<int>>>> dp;
std::vector<int>::iterator it;
int f(int a, int b, int c, int last) {
if(dp[a][b][c][last] != -1) {
return dp[a][b][c][last];
} else if(a + b + c == n) {
return 0;
}
int res = INF;
if(a != idxs[0].size() && last != 0) {
int cost = 0;
cost += std::max(0, pre[0][a][1] - b);
cost += std::max(0, pre[0][a][2] - c);
res = std::min(res, f(a + 1, b, c, 0) + cost);
}
if(b != idxs[1].size() && last != 1) {
int cost = 0;
cost += std::max(0, pre[1][b][0] - a);
cost += std::max(0, pre[1][b][2] - c);
res = std::min(res, f(a, b + 1, c, 1) + cost);
}
if(c != idxs[2].size() && last != 2) {
int cost = 0;
cost += std::max(0, pre[2][c][0] - a);
cost += std::max(0, pre[2][c][1] - b);
res = std::min(res, f(a, b, c + 1, 2) + cost);
}
return dp[a][b][c][last] = res;
}
void solve() {
std::string s;
std::cin >> n >> s;
for(int i = 0; i < n; i++) {
if(s[i] == 'R') {
idxs[0].emplace_back(i);
} else if(s[i] == 'G') {
idxs[1].emplace_back(i);
} else {
idxs[2].emplace_back(i);
}
}
dp.assign(idxs[0].size() + 1, std::vector (idxs[1].size() + 1, std::vector (idxs[2].size() + 1, std::vector (4, -1))));
for(int i = 0; i < idxs[0].size(); i++) {
pre[0][i][1] = std::upper_bound(all(idxs[1]), idxs[0][i]) - idxs[1].begin();
pre[0][i][2] = std::upper_bound(all(idxs[2]), idxs[0][i]) - idxs[2].begin();
}
for(int i = 0; i < idxs[1].size(); i++) {
pre[1][i][0] = std::upper_bound(all(idxs[0]), idxs[1][i]) - idxs[0].begin();
pre[1][i][2] = std::upper_bound(all(idxs[2]), idxs[1][i]) - idxs[2].begin();
}
for(int i = 0; i < idxs[2].size(); i++) {
pre[2][i][0] = std::upper_bound(all(idxs[0]), idxs[2][i]) - idxs[0].begin();
pre[2][i][1] = std::upper_bound(all(idxs[1]), idxs[2][i]) - idxs[1].begin();
}
int ans = f(0, 0, 0, 3);
if(ans != INF) {
std::cout << ans;
} else {
std::cout << -1;
}
return;
}
signed main() {
#ifdef LOCAL
freopen("in.txt", "r", stdin);
freopen("out.txt", "w", stdout);
#endif
std::ios_base::sync_with_stdio(false);
std::cin.tie(NULL); std::cout.tie(NULL);
int t = 1; //std::cin >> t;
for(int i = 1; i <= t; i++) {
solve();
}
return 0;
}
