/*
_____________________.
|____________________| $$\ $$\ $$\ $$\
|____________________| $$ | \__| $$ | $$ |
|____________________| $$$$$$\ $$$$$$\ $$$$$$\ $$$$$$$\ $$$$$$$\ $$$$$$\ $$\ $$$$$$\ $$$$$$$\ $$$$$$\ $$$$$$$\
|____________________| \_$$ _| $$ __$$\ \____$$\ $$ __$$\ $$ _____| $$ __$$\ $$ |$$ __$$\ $$ __$$\\_$$ _| $$ _____|
|____________________| $$ | $$ | \__|$$$$$$$ |$$ | $$ |\$$$$$$\ $$ | \__|$$ |$$ / $$ |$$ | $$ | $$ | \$$$$$$\
$$ |$$\ $$ | $$ __$$ |$$ | $$ | \____$$\ $$ | $$ |$$ | $$ |$$ | $$ | $$ |$$\ \____$$\
\$$$$ |$$ | \$$$$$$$ |$$ | $$ |$$$$$$$ | $$ | $$ |\$$$$$$$ |$$ | $$ | \$$$$ |$$$$$$$ |
へ ♡ \____/ \__| \_______|\__| \__|\_______/ \__| \__| \____$$ |\__| \__| \____/ \_______/
૮ - ՛) $$\ $$ |
/ ⁻ ៸| \$$$$$$ | are human rights :3
乀 (ˍ,ل ل \______/
*/
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,
tree_order_statistics_node_update>;
template <typename T>
using ordered_multiset = tree<pair<T, long long>, null_type,
less<pair<T, long long>>, rb_tree_tag,
tree_order_statistics_node_update>;
using namespace std;
using namespace __gnu_pbds;
#define ll long long
#define ull unsigned long long
#define ld long double
#define inf (ll)2e18+4
#define pb push_back
#define se second
#define fi first
#define endl '\n'
#define mp make_pair
#define pll pair<ll,ll>
#define kth_smallest find_by_order
#define num_of_smaller order_of_key
#define fori(x) for(ll i=0;i<x;i++)
#define forj(y) for(ll j=0;j<y;j++)
#define fork(z) for(ll k=0;k<z;k++)
#define DEBUG
#ifdef DEBUG
#define show(x) cerr<<#x<<" is "<<x<<endl;
#define show2(x,y) cerr<<#x<<" is "<<x<<" "<<#y<<" is "<<y<<endl;
#define show3(x,y,z) cerr<<#x<<" is "<<x<<" "<<#y<<" is "<<y<<" "<<#z<<" is "<<z<<endl;
#define show4(x,y,z,a) cerr<<#x<<" is "<<x<<" "<<#y<<" is "<<y<<" "<<#z<<" is "<<z<<" "<<#a<<" is "<<a<<endl;
#define show_vec(a) for(auto &i:a)cerr<<i<<" ";cerr<<endl;
#define skillissue cerr<<"your code is running\n";
#define getchar_unlocked _getchar_nolock // comment before submission
#else
#define show(x)
#define show2(x,y)
#define show3(x,y,z)
#define show4(x,y,z,a)
#define show_vec(a)
#define skillissue
#endif
/*
inline int readint() {
int x=0; char ch=getchar_unlocked(); bool s=1;
while(ch<'0'||ch>'9'){if(ch=='-')s=0;ch=getchar_unlocked();}
while(ch>='0'&&ch<='9'){x=(x<<3)+(x<<1)+ch-'0';ch=getchar_unlocked();}
return s?x:-x;
}
*/
const ll maxn=63;
ll s[maxn];
ll at[3][maxn];
ll n;
ll mem[maxn][maxn][maxn][3];
// G Y
ll dp(ll i,ll j,ll k,ll last){
ll l=n-i-j-k;
if(j<0 or k<0 or l<0){return inf;}
if(i==n)return 0;
if(mem[i][j][k][last]!=-1)return mem[i][j][k][last];
if(last==0)
return mem[i][j][k][last]=min({abs(at[2][l]-i)+dp(i+1,j,k,2), abs(at[1][k]-i)+dp(i+1,j,k-1,1)});
else if(last==1)
return mem[i][j][k][last]=min({abs(at[2][l]-i)+dp(i+1,j,k,2), abs(at[0][j]-i)+dp(i+1,j-1,k,0)});
else if(last==2)
return mem[i][j][k][last]=min({abs(at[1][k]-i)+dp(i+1,j,k-1,1), abs(at[0][j]-i)+dp(i+1,j-1,k,0)});
else
return min({abs(at[1][k]-i)+dp(i+1,j,k-1,1), abs(at[0][j]-i)+dp(i+1,j-1,k,0),abs(at[2][l]-i)+dp(i+1,j,k,2)});
}
int main(){
ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
cin>>n;
string ss;cin>>ss;
memset(mem,-1,sizeof(mem));
fori(n){
if(ss[i]=='R')s[i]=0;
else if(ss[i]=='G')s[i]=1;
else s[i]=2;
}
ll cnt[3];
forj(3){
ll c=0;
for(ll i=n-1;i>=0;i--){
if(s[i]==j){
c++;
at[j][c]=i;
}
}
cnt[j]=c;
at[j][0]=inf;
}
cout<<(dp(0,cnt[0],cnt[1],4)>=(1e10)?-1:dp(0,cnt[0],cnt[1],4)/2);
}
