#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define all(x) (x).begin(),(x).end()
#define lb(x) ((x)&-(x))
#define IOS cin.tie(nullptr)->sync_with_stdio(0);
constexpr int maxN = 4e2+5;
int n,dp[2][maxN][maxN][3],others[3][maxN],pos[3][maxN],cnt[3];
string s;
inline int cal(int v1,int v2,int v3,int nxt){
//cout<<v1<<' '<<v2<<' '<<v3<<' '<<nxt<<'\n';
int ret = 0,p = 0;
if(nxt==0)p = pos[0][v1+1];
else if(nxt==1)p = pos[1][v2+1];
else if(nxt==2)p = pos[2][v3+1];
ret += max(0,others[0][p]-v1-(nxt==0));
ret += max(0,others[1][p]-v2-(nxt==1));
ret += max(0,others[2][p]-v3-(nxt==2));
return ret;
}
inline void chmin(int& a,int b){
a=min(a,b);
}
int main(){
IOS
cin>>n>>s;
s=' '+s;
for(int i = 1;i<=n;i++){
int v = (s[i]=='R'?0:(s[i]=='G'?1:2));
for(int j = 0;j<3;j++)others[j][i]=others[j][i-1];
others[v][i]++;
pos[v][++cnt[v]]=i;
}
for(int j = 0;j<=cnt[1];j++){
for(int k = 0;k<=cnt[2];k++){
for(int nxt = 0;nxt<3;nxt++){
dp[0][j][k][nxt] = 1e9;
}
}
}
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for(int i = 0;i<n;i++){
for(int j = 0;j<=cnt[1];j++){
for(int k = 0;k<=cnt[2];k++){
for(int nxt = 0;nxt<3;nxt++){
dp[i&1^1][j][k][nxt] = 1e9;
}
}
}
for(int j = 0;j<=cnt[1]&&j<=i;j++){
for(int k = 0;k<=cnt[2]&&j+k<=i;k++)if(i-j-k<=cnt[0]){
for(int lst = 0;lst<3;lst++){
if(lst!=0&&i-j-k<cnt[0])chmin(dp[i&1^1][j][k][0],dp[i&1][j][k][lst] + cal(i-j-k,j,k,0));
if(lst!=1&&j<cnt[1])chmin(dp[i&1^1][j+1][k][1],dp[i&1][j][k][lst] + cal(i-j-k,j,k,1));
if(lst!=2&&k<cnt[2])chmin(dp[i&1^1][j][k+1][2],dp[i&1][j][k][lst] + cal(i-j-k,j,k,2));
}
}
}
}
int ans = *min_element(dp[n&1][cnt[1]][cnt[2]],dp[n&1][cnt[1]][cnt[2]]+3);
if(ans>n*n)cout<<"-1\n";
else cout<<ans<<'\n';
}
