#include <iostream>
#include <iomanip>
#include <vector>
#include <cmath>
#include <algorithm>
#include <set>
#include <queue>
#include <map>
#include <stack>
#include <bitset>
#include <string>
#include <cstring>
#include <iterator>
#include <random>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef long double ld;
const ll dim = 5*1e3+7;
//const ll mod = 1e9 + 7;
const ll inf = 1e18 + 77;
#define endl "\n"
#define fi first
#define pb push_back
#define se second
#define vll vector<ll>
ll n, m;
pll a[dim];
ll b[dim];
ll check(string &s){
ll fl=1;
for(int j=1; j<s.length(); j++){
if(s[j]==s[j-1])fl=0;
}
return fl;
}
int main() {
ll k, t, u0, v0;
string s;
cin>>n;
cin>>s;
string s1=s;
ll cnt1=0;
ll cnt2=0;
ll cnt3=0;
for(int i=1; i<=n; i++){
if(s[i-1]=='G')cnt1++;
else if(s[i-1]=='R')cnt2++;
else cnt3++;
}
if(cnt1>(n+1)/2 || cnt2>(n+1)/2 || cnt3>(n+1)/2){
cout<<-1<<endl;
return 0;
}
ll ans=0;
for(int i=1; i<n; i++){
if(s[i]==s[i-1]){
for(int j=i+1; j<n; j++){
if(s[j]!=s[i]){
ans+=abs(j-i);
swap(s[j], s[i]);
break;
}
}
}
}
ll ans1=inf;
if(check(s))ans1=ans;
s=s1;
ans=0;
for(int i=1; i<n; i++){
if(s[i]!=s[0]){
s=string(1, s[i])+s.substr(0, i)+s.substr(i+1, n-i+1);
ans+=i;
break;
}
}
for(int i=1; i<n; i++){
if(s[i]==s[i-1]){
for(int j=i+1; j<n; j++){
if(s[j]!=s[i]){
ans+=abs(j-i);
swap(s[j], s[i]);
break;
}
}
}
}
if(check(s))ans1=min(ans1, ans);
if(ans1==inf){
cout<<-1<<endl;
}
else{
cout<<ans1<<endl;
}
// cout<<s<<endl;
return 0;
}
