#include<bits/stdc++.h>
using namespace std;
const int INF=1e9;
int dp[2][404][404][3],pos[404][3];
int main()
{
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n;
string s;
cin>>n>>s;
s=' '+s;
int cnta=0,cntb=0,cntc=0;
for(int i=1;i<=n;i++)
{
if(s[i]=='R') pos[++cnta][0]=i;
if(s[i]=='G') pos[++cntb][1]=i;
if(s[i]=='Y') pos[++cntc][2]=i;
}
for(int i=1;i<=n;i++)
{
int a=i%2,b=1-a;
for(int j=0;j<=i&&j<=cnta;j++) for(int k=0;k+j<=i&&k<=cntb;k++)
{
dp[a][j][k][0]=dp[a][j][k][1]=dp[a][j][k][2]=INF;
if(j>0) dp[a][j][k][0]=min(dp[a][j][k][0],min(dp[b][j-1][k][1],dp[b][j-1][k][2])+abs(pos[j][0]-i));
if(k>0) dp[a][j][k][1]=min(dp[a][j][k][1],min(dp[b][j][k-1][0],dp[b][j][k-1][2])+abs(pos[k][1]-i));
if(j+k<i&&i-j-k<=cntc) dp[a][j][k][2]=min(dp[a][j][k][2],min(dp[b][j][k][0],dp[b][j][k][1])+abs(pos[i-j-k][2]-i));
}
}
int ans=min(dp[n%2][cnta][cntb][0],min(dp[n%2][cnta][cntb][1],dp[n%2][cnta][cntb][2]));
if(ans>=INF) cout<<-1;
else cout<<ans/2;
}
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