#pragma GCC optimize("Ofast")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define pb push_back
#define ld long double
#define fi first
#define se second
#define ll long long
#define ss(x) (int) x.size()
#define mp make_pair
#define FOR(i, n) for(int i = 1; n >= i; ++i)
using namespace std;
using namespace __gnu_pbds;
const int inf = 1e9 + 1;
int dp[405][405][405][3];
int n;
string s;
int cnt[405][3];
int t[405];
vector <int> v[3];
int ile[3];
int main() {
cin.tie(0);
ios_base::sync_with_stdio(0);
cin >> n >> s;
for(int i = 0; i < n; ++i) {
if(s[i] == 'R')
t[i + 1] = 0;
if(s[i] == 'G')
t[i + 1] = 1;
if(s[i] == 'Y')
t[i + 1] = 2;
}
for(int i = 0; i < 3; ++i) {
v[i].pb(0);
}
for(int i = 1; i <= n; ++i) {
for(int j = 0; j < 3; ++j)
cnt[i][j] = cnt[i - 1][j];
cnt[i][t[i]]++;
v[t[i]].pb(i);
}
int R = cnt[n][0];
int G = cnt[n][1];
int Y = cnt[n][2];
for(int r = 0; r <= R; ++r)
for(int g = 0; g <= G; ++g)
for(int y = 0; y <= Y; ++y)
for(int z = 0; z < 3; ++z)
dp[r][g][y][z] = inf;
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
int maxi = max({R, G, Y});
if(maxi > (n + 1) / 2) {
cout << -1 << endl;
return 0;
}
for(int r = 0; r <= R; ++r)
for(int g = 0; g <= G; ++g)
for(int y = 0; y <= Y; ++y) {
for(int last = 0; last < 3; ++last) {
int ans = dp[r][g][y][last];
ile[0] = r;
ile[1] = g;
ile[2] = y;
for(int i = 0; i < 3; ++i) {
if(i == last || ile[i] + 1 == ss(v[i]))
continue;
int pos = v[i][ile[i] + 1];
int add = -1 + pos - r - g - y;
for(int j = 0; j < 3; ++j) {
int ost = v[j][ile[j]];
add += max(0, cnt[ost][j] - cnt[pos][j]);
}
//cout << r << " " << g << " " << y << " " << " " << i << " " << ans << endl;
ile[i]++;
dp[ile[0]][ile[1]][ile[2]][i] = min(dp[ile[0]][ile[1]][ile[2]][i], add + ans);
ile[i]--;
}
}
}
int best = inf;
for(int i = 0; i < 3; ++i)
best = min(best, dp[R][G][Y][i]);
cout << best;
return 0;
}
