#include <bits/stdc++.h>
#pragma GCC optimize("unroll-loops")
using namespace std;
#define ll long long
#define ff first
#define ss second
#define pb push_back
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
#define N 405
#define INF 0x3f3f3f3f
#define MOD 1000000007LL
char c;
int dp[N][N][N][3];
int a[N];
vector<int> app[3];
int i[3];
int cost(int k){
i[k]--;
int res = INF;
for(int r = 0; r < 3; ++r)
if(r != k)
res = min(res, dp[i[0]][i[1]][i[2]][r]);
int v = app[k][i[k]];
i[k]++;
int w = v;
for(int j = 0; j < 3; j++){
int a = upper_bound(app[j].begin(), app[j].begin() + i[j], v) - app[j].begin();
w += max(0, i[j] - a);
}
return res + abs(i[0] + i[1] + i[2] - w);
}
int s[3];
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n;
cin>>n;
for(int i = 1; i <= n; i++){
cin>>c;
if(c == 'R')
app[0].push_back(i);
else if(c == 'G')
app[1].push_back(i);
else
app[2].push_back(i);
}
for(int i = 0; i < 3; ++i)
s[i] = app[i].size();
for(i[0] = 0; i[0] <= s[0]; ++i[0]){
for(i[1] = 0; i[1] <= s[1]; ++i[1]){
for(i[2] = 0; i[2] <= s[2]; ++i[2]){
for(int k = 0; k < 3; ++k){
if(i[0] + i[1] + i[2] == 0)
dp[i[0]][i[1]][i[2]][k] = 0;
else if(i[k] == 0)
dp[i[0]][i[1]][i[2]][k] = INF;
else
dp[i[0]][i[1]][i[2]][k] = cost(k);
}
}
}
}
int res = min(dp[s[0]][s[1]][s[2]][0], min(dp[s[0]][s[1]][s[2]][1], dp[s[0]][s[1]][s[2]][2]));
if(res == 0x3f3f3f3f)
cout<<-1;
else
cout<<res;
}
