#include<bits/stdc++.h>
using namespace std;
const int N = 402, INF = 1e9;
int n, a[N], dp[2][3][N][N], pref[3][N][N];
vector<int> kth_char[3];
string s;
int main() {
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin>>n>>s;
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;
}
for(int c=0; c<3; ++c) {
kth_char[c].push_back(-1);
}
for(int i=0; i<n; ++i) {
kth_char[a[i]].push_back(i);
}
for(int c=0; c<3; ++c) {
for(int i=0; i<(int)kth_char[c].size(); ++i) {
int cnt = i;
for(int j=0; j<n; ++j) {
if(j!=0) pref[c][i][j] = pref[c][i][j-1];
if(cnt>0 && a[j]==c) {
pref[c][i][j] ++;
cnt --;
}
}
}
}
for(int c=0; c<3; ++c) {
for(int i=0; i<=n; ++i) {
for(int j=0; j<=n; ++j) {
dp[1][c][i][j] = INF;
}
}
dp[1][c][0][0] = 0;
}
for(int it=0; it<n; ++it) {
for(int c=0; c<3; ++c) {
for(int i=0; i<(int)kth_char[0].size(); ++i) {
for(int j=0; j<(int)kth_char[1].size(); ++j) {
int k = it+1 - i - j;
dp[it%2][c][i][j] = INF;
if(k < 0 || k >= (int)kth_char[2].size()) continue;
if(c==0 && i>0) {
int pos = kth_char[c][i];
dp[it%2][c][i][j] = min(dp[(it+1)%2][1][i-1][j], dp[(it+1)%2][2][i-1][j]) + pos + 1 - (pref[0][i][pos] + pref[1][j][pos] + pref[2][k][pos]) ;
}
if(c==1 && j>0) {
int pos = kth_char[c][j];
dp[it%2][c][i][j] = min(dp[(it+1)%2][0][i][j-1], dp[(it+1)%2][2][i][j-1]) + pos + 1 - (pref[0][i][pos] + pref[1][j][pos] + pref[2][k][pos]) ;
}
if(c==2 && k>0) {
int pos = kth_char[c][k];
dp[it%2][c][i][j] = min(dp[(it+1)%2][0][i][j], dp[(it+1)%2][1][i][j]) + pos + 1 - (pref[0][i][pos] + pref[1][j][pos] + pref[2][k][pos]) ;
}
}
}
}
}
int ans = INF;
for(int c=0; c<3; ++c) {
ans = min(ans, dp[(n-1)%2][c][kth_char[0].size()-1][kth_char[1].size()-1]);
}
if(ans < INF) cout<<ans;
else cout<<-1;
}
