#include<bits/stdc++.h>
using namespace std;
#define rc(x) return cout<<x<<endl,0
#define pb push_back
#define dbg(x) cout << #x << '=' << x << '\n';
#define ll long long
#define sz size()
#define x first
#define y second
#define pi pair <int, int>
#define pii pair <int, pi>
#define vi vector <int>
const ll mod = 1e9 + 7;
int n, dp[405][405][405][3], cnt[3];
string s;
vector<pii> v[3];
int get(char c){
if(c=='R') return 0;
else if(c=='G') return 1;
else return 2;
}
int dst(pii a, pii b){
return abs(a.x-b.x)+abs(a.y.x-b.y.x)+abs(a.y.y-b.y.y);
}
int32_t main(){
ios_base :: sync_with_stdio(0); cin.tie(); cout.tie();
cin>>n>>s;
for(int i=0; i<n; i++){
int x=get(s[i]);
cnt[x]++;
v[x].pb({cnt[0], {cnt[1], cnt[2]}});
}
int rr=v[0].sz, gg=v[1].sz, yy=v[2].sz;
if(max(rr, max(gg, yy))>(n+1)/2) return cout<<"-1", 0;
for(int r=0; r<=rr; r++){
for(int g=0; g<=gg; g++){
for(int y=0; y<=yy; y++){
int sum=r+g+y;
if(sum==0) continue;
dp[r][g][y][0]=dp[r][g][y][1]=dp[r][g][y][2]=1e9;
if(r) dp[r][g][y][0]=min(dp[r-1][g][y][1], dp[r-1][g][y][2])+dst(v[0][r-1], {r, {g, y}});
if(g) dp[r][g][y][1]=min(dp[r][g-1][y][2], dp[r][g-1][y][0])+dst(v[1][g-1], {r, {g, y}});
if(y) dp[r][g][y][2]=min(dp[r][g][y-1][0], dp[r][g][y-1][1])+dst(v[2][y-1], {r, {g, y}});
}
}
}
cout<<min(dp[rr][gg][yy][0], min(dp[rr][gg][yy][1], dp[rr][gg][yy][2]))/2;
}
