#include<bits/stdc++.h>
using namespace std;
#define ll int
#define fi first
#define se second
#define p(x,y) pair<ll,ll>(x,y)
#define BIT(i,x) ((x>>i)&1)
#define MASK(x) (1<<x)
#define ld long double
#define __builtin_popcount __builtin_popcountll
#define pll pair<ll,ll>
template<class T1,class T2>
bool maximize(T1 &x,const T2 &y)
{
if(x<y)
{
x=y;
return 1;
}
return 0;
}
template<class T1,class T2>
bool minimize(T1 &x,const T2 &y)
{
if(x>y)
{
x=y;
return 1;
}
return 0;
}
void online()
{
std::ios_base::sync_with_stdio(0);
cin.tie(0);
#ifdef thuc
freopen("input.INP","r",stdin);
freopen("output.OUT","w",stdout);
#else
#endif
}
const ll N=400;
ll f[N][N][N][3],n;
vector<ll> vt[3];
ll lg[4][4][N+2][N+2];
string s;
void solve(ll x,ll y)
{
for(int i=0;i<vt[x].size();i++)
{
for(int j=0;j<=vt[y].size();j++)
{
if(j>0)
lg[x][y][i][j]+=(vt[x][i]>vt[y][j-1]);
lg[x][y][i][j+1]+=lg[x][y][i][j];
}
}
}
ll dp(ll i,ll j,ll k,ll pre)
{
if(i==vt[0].size()&&j==vt[1].size()&&k==vt[2].size()) return 0;
ll &res=f[i][j][k][pre];
if(res!=-1) return res;
res=1e9;
if(pre!=0&&i<vt[0].size())
{
minimize(res,dp(i+1,j,k,0)+vt[0][i]-i-lg[0][1][i][j]-lg[0][2][i][k]);
}
if(pre!=1&&j<vt[1].size())
{
minimize(res,dp(i,j+1,k,1)+vt[1][j]-j-lg[1][0][j][i]-lg[1][2][j][k]);
}
if(pre!=2&&k<vt[2].size())
{
minimize(res,dp(i,j,k+1,2)+vt[2][k]-k-lg[2][0][k][i]-lg[2][1][k][j]);
}
return res;
}
int main()
{
online();
cin>>n;
cin>>s;
for(int i=0;i<n;i++)
{
if(s[i]=='R') vt[0].push_back(i);
if(s[i]=='G') vt[1].push_back(i);
if(s[i]=='Y') vt[2].push_back(i);
}
solve(0,1);
solve(0,2);
solve(1,0);
solve(1,2);
solve(2,0);
solve(2,1);
memset(f,-1,sizeof f);
ll z=min(dp(0,0,0,0),min(dp(0,0,0,1),dp(0,0,0,2)));
if(z>=1e9)
{
z=-1;
}
cout<<z;
}
