#include <bits/stdc++.h>
using namespace std;
template<typename T>
void out(T x) { cout << x << endl; exit(0); }
#define watch(x) cout << (#x) << " is " << (x) << endl
using ll = long long;
const int maxn = 444;
const int inf = 1e9;
int n;
string s;
int a[maxn];
int dp[maxn][maxn][maxn][3];
vector<int> inds[maxn];
int prevs[3][maxn]; // prev[j][i]: # of j's in a[0...i-1]
void upd(int &x, int y) {
x=min(x,y);
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin>>n;
cin>>s;
for (int i=0; i<n; i++) {
if (s[i]=='R') {
a[i]=0;
} else if (s[i]=='G') {
a[i]=1;
} else if (s[i]=='Y') {
a[i]=2;
} else assert(false);
}
for (int i=0; i<n; i++) {
inds[a[i]].push_back(i);
for (int j=0; j<3; j++) {
prevs[j][i] = inds[j].size();
}
}
for (int a=0; a<=(int)inds[0].size(); a++) {
for (int b=0; b<=(int)inds[1].size(); b++) {
for (int c=0; c<=(int)inds[2].size(); c++) {
for (int last=0; last<3; last++) {
dp[a][b][c][last] = inf;
}
}
}
}
for (int last=0; last<3; last++) {
dp[0][0][0][last] = 0;
}
for (int a=0; a<=(int)inds[0].size(); a++) {
for (int b=0; b<=(int)inds[1].size(); b++) {
for (int c=0; c<=(int)inds[2].size(); c++) {
for (int last=0; last<3; last++) {
// place R
// to place it, need to swap past all the G and Y's
if (a<(int)inds[0].size() && last!=0) {
int p = inds[0][a];
int cur = dp[a][b][c][last] + max(0, prevs[1][p]-b) + max(0, prevs[2][p]-c);
upd(dp[a+1][b][c][0], cur);
}
// place G
if (b<(int)inds[1].size() && last!=1) {
int p = inds[1][b];
int cur = dp[a][b][c][last] + max(0, prevs[0][p]-a) + max(0, prevs[2][p]-c);
upd(dp[a][b+1][c][1], cur);
}
// place Y
if (c<(int)inds[2].size() && last!=2) {
int p = inds[2][c];
int cur = dp[a][b][c][last] + max(0, prevs[0][p]-a) + max(0, prevs[1][p]-b);
upd(dp[a][b][c+1][2], cur);
}
}
}
}
}
int res = inf;
for (int i=0; i<3; i++) {
res = min(res, dp[(int)inds[0].size()][(int)inds[1].size()][(int)inds[2].size()][i]);
}
if (res > inf/2) res = -1;
out(res);
return 0;
}
