// File gvif3.cpp created on 02.10.2025 at 11:20:37
#include <bits/stdc++.h>
using i64 = long long;
#ifdef DEBUG
#include "/home/ahmetalp/Desktop/Workplace/debug.h"
#else
#define debug(...) void(23)
#endif
constexpr int inf = int(1E9);
template<typename T>
bool chmin(T& a, T b) {
if (a > b) {
a = b;
return true;
}
return false;
}
int main() {
std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);
int N;
std::cin >> N;
std::string S;
std::cin >> S;
std::vector<int> A(N);
for (int i = 0; i < N; ++i) {
if (S[i] == 'R') {
A[i] = 0;
} else if (S[i] == 'G') {
A[i] = 1;
} else {
A[i] = 2;
}
}
std::vector<std::vector<int>> ids(3);
std::vector<std::vector<int>> pre(3, std::vector<int>(N + 1));
for (int i = 0; i < N; ++i) {
ids[A[i]].emplace_back(i);
pre[0][i + 1] = pre[0][i] + (A[i] == 0);
pre[1][i + 1] = pre[1][i] + (A[i] == 1);
pre[2][i + 1] = pre[2][i] + (A[i] == 2);
}
int R = pre[0][N];
int G = pre[1][N];
int B = pre[2][N];
std::vector f(R + 1, std::vector(G + 1, std::vector(B + 1, std::vector<int>(3, inf))));
f[0][0][0][0] = 0;
f[0][0][0][1] = 0;
f[0][0][0][2] = 0;
for (int i = 0; i <= R; ++i) {
for (int j = 0; j <= G; ++j) {
for (int k = 0; k <= B; ++k) {
for (int x = 0; x < 3; ++x) {
for (int y = 0; y < 3; ++y) {
if (x == y) {
continue;
}
int ni = i + (y == 0);
int nj = j + (y == 1);
int nk = k + (y == 2);
if (ni > R || nj > G || nk > B) {
continue;
}
int id = ids[y][(y == 0 ? i : (y == 1 ? j : k))];
chmin(f[ni][nj][nk][y], f[i][j][k][x] + std::max(0, pre[0][id] - i) + std::max(0, pre[1][id] - j) + std::max(0, pre[2][id] - k));
}
}
}
}
}
int ans = std::min({f[R][G][B][0], f[R][G][B][1], f[R][G][B][2]});
if (ans == inf) {
ans = -1;
}
std::cout << ans << '\n';
return 0;
}
