#include<bits/stdc++.h>
using namespace std;
using ll = long long;
struct BIT{
ll n;
vector<ll> a;
BIT(ll N):n(N),a(N+1){}
void add(ll i, ll x){
for(;i<=n;i+=i&-i) a[i] += x;
}
ll get(ll i){
ll x = 0;
for(;i;i-=i&-i) x +=a[i];
return x;
}
};
int main(){
ios::sync_with_stdio(0);cin.tie(0);
ll n;
string s;
cin >> n >> s;
vector<ll> cnt(3,0);
vector<ll> a(n);
vector<vector<ll>> ps(3);
for(ll i = 0;i<n;i++) {
a[i] = s[i]=='R'?0:s[i]=='G'?1:2;
cnt[a[i]]++;
ps[a[i]].push_back(i+1);
}
for(ll i : cnt){
if (i>(n+1)/2){
cout << "-1\n";
return 0;
}
}
vector<vector<ll>> c(3,vector<ll>(n+1,0));
for(ll i = 0;i<n;i++){
for(ll j = 0;j<3;j++){
c[j][i+1] = c[j][i] + (a[i]==j);
}
}
vector<vector<vector<vector<ll>>>> dp(3,vector<vector<vector<ll>>>(n+5,vector<vector<ll>>(n+5,vector<ll>(n+5,1e18))));
dp[0][0][0][0] = dp[1][0][0][0] = dp[2][0][0][0] = 0;
for(ll i = 0;i<=cnt[0];i++){
for(ll j = 0;j<=cnt[1];j++){
for(ll k = 0;k<=cnt[2];k++){
ll pos = i+j+k;
if(i>0) dp[0][i][j][k] = min(dp[0][i][j][k],min(dp[1][i-1][j][k],dp[2][i-1][j][k])+max(c[0][ps[0][i-1]]-i,0ll)+max(c[1][ps[0][i-1]]-j,0ll)+max(c[2][ps[0][i-1]]-k,0ll));
if(j>0) dp[1][i][j][k] = min(dp[1][i][j][k],min(dp[0][i][j-1][k],dp[2][i][j-1][k])+max(c[0][ps[1][j-1]]-i,0ll)+max(c[1][ps[1][j-1]]-j,0ll)+max(c[2][ps[1][j-1]]-k,0ll));
if(k>0) dp[2][i][j][k] = min(dp[2][i][j][k],min(dp[0][i][j][k-1],dp[1][i][j][k-1])+max(c[0][ps[2][k-1]]-i,0ll)+max(c[1][ps[2][k-1]]-j,0ll)+max(c[2][ps[2][k-1]]-k,0ll));
}
}
}
ll ans = 1e18;
for(ll i = 0;i<3;i++) ans = min(ans,dp[i][cnt[0]][cnt[1]][cnt[2]]);
cout << ans << "\n";
}
