#include<bits/stdc++.h>
//#pragma GCC optimize("Ofast,unroll-loops")
//#pragma GCC target("avx2,popcnt,lzcnt,abm,bmi,bmi2,fma,tune=native")
#define pii pair<int, int>
#define fi first
#define se second
#define pb push_back
//#define int long long
//#define ld long double
using namespace std;
using ll=long long;
const int inf = 1e9;
const int MN =400+5;
const int mod=1e9+7;
int n;
string s;
int cnt[MN][MN][4][4];
int dp[MN][MN][MN][4];
int a[MN],b[MN],c[MN];
void solve() {
cin>>n>>s;
// s="#"+s;
int n1=0,n2=0,n3=0;
for (int i=0; i<n; i++) {
if(s[i]=='R') a[++n1]=i;
else if(s[i]=='Y') b[++n2]=i;
else c[++n3]=i;
}
int k;
k=0;
for (int i=1; i<=n1; i++) {
k=0;
for (int j=1; j<=n2; j++) {
if(b[j]>a[i]) k++;
cnt[i][j][1][2]=k;
// cout<<cnt[i][j][1][2]<<" ";
}
}
k=0;
for (int i=1; i<=n1; i++) {
k=0;
for (int j=1; j<=n3; j++) {
if(c[j]>a[i]) k++;
cnt[i][j][1][3]=k;
}
}
k=0;
for (int i=1; i<=n2; i++) {
k=0;
for (int j=1; j<=n1; j++) {
if(a[j]>b[i]) k++;
cnt[i][j][2][1]=k;
}
}
k=0;
for (int i=1; i<=n2; i++) {
k=0;
for (int j=1; j<=n3; j++) {
if(c[j]>b[i]) k++;
cnt[i][j][2][3]=k;
}
}
k=0;
for (int i=1; i<=n3; i++) {
k=0;
for (int j=1; j<=n1; j++) {
if(a[j]>c[i]) k++;
cnt[i][j][3][1]=k;
}
}
k=0;
for (int i=1; i<=n3; i++) {
k=0;
for (int j=1; j<=n2; j++) {
if(b[j]>c[i]) k++;
cnt[i][j][3][2]=k;
}
}
for (int i=0; i<=n; i++) {
for (int j=0; j<=n; j++) {
for (int l=0; l<=n; l++) {
dp[i][j][l][1]=dp[i][j][l][2]=dp[i][j][l][3]=inf;
}
}
}
dp[0][0][0][1]=dp[0][0][0][2]=dp[0][0][0][3]=0;
for (int i=0; i<=n1; i++) {
for (int j=0; j<=n2; j++) {
for (int l=0; l<=n3; l++) {
if(i>0) dp[i][j][l][1] = min(dp[i-1][j][l][2],dp[i-1][j][l][3]) + cnt[i][j][1][2] + cnt[i][l][1][3];
if(j>0) dp[i][j][l][2] = min(dp[i][j-1][l][1],dp[i][j-1][l][3]) + cnt[j][i][2][1] + cnt[j][l][2][3];
if(l>0) dp[i][j][l][3] = min(dp[i][j][l-1][2],dp[i][j][l-1][1]) + cnt[l][i][3][1] + cnt[l][j][3][2];
}
}
}
int ans=min({dp[n1][n2][n3][1], dp[n1][n2][n3][2], dp[n1][n2][n3][3]});
if(ans>=inf) cout<<-1;
else cout<<ans;
}
signed main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
// freopen(".inp", "r", stdin);
// freopen(".out", "w", stdout);
int t=1;
// cin>>t;
while(t--) {
solve();
}
}
