#include <iostream>
#include <bits/stdc++.h>
#include <cmath>
#include <algorithm>
#include <vector>
#include <deque>
#include <set>
#include <stack>
#include <string>
#include <map>
#include <queue>
#define int long long
using namespace std;
const long long oo = 1000000000000000000;
long long sum, ans = 0, mx = 0, mn = 1000000000, num, pos;
/*
ViHHiPuh
(( `'-""``""-'` ))
)-__-_.._-__-(
/ --- (o _ o) --- \
\ .-* ( .0. ) *-. /
_'-. ,_ '=' _, .-'_
/ `;#'#'# - #'#'#;` \
\_)) -----'#'----- ((_/
# --------- #
'# ------- ------ #'
/..-'# ------- #'-.\
_\...-\'# -- #'/-.../_
((____)- '#' -(____))
cout << fixed << setprecision(6) << x;
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
freopen ( "sum.in", "r", stdin )
*/
int dp[ 401 ][ 401 ][ 401 ][ 3 ];
vector <pair<int, pair<int, int> > > v, vv, v1;
int f( int a, int b ) {
return min( a, b );
}
main () {
int n;
cin >> n;
string s;
cin >> s;
for ( int i = 0; i < s.size(); i ++ ) {
if ( s[ i ] == 'R' ) {
sum ++;
v.push_back( { i, make_pair( vv.size(), v1.size() ) } );
}
if ( s[ i ] == 'Y' ) {
num ++;
v1.push_back( { i, make_pair( v.size(), vv.size() ) } );
}
if ( s[ i ] == 'G' ) {
mx ++;
vv.push_back( { i, make_pair( v.size(), v1.size() ) } );
}
}
if ( max( mx, max( num, sum ) ) > ( n + 1 ) / 2 ) {
cout << "-1";
return 0;
}
for ( int i = 0; i <= sum; i ++ ) {
for ( int j = 0; j <= mx; j ++ ) {
for ( int k = 0; k <= num; k ++ ) {
if ( i + j + k == 0 )
continue;
dp[ i ][ j ][ k ][ 1 ] = dp[ i ][ j ][ k ][ 2 ] = dp[ i ][ j ][ k ][ 3 ] = oo;
}
}
}
for ( int a = 0; a <= sum; a ++ ) {
for ( int b = 0; b <= mx; b ++ ) {
for ( int c = 0; c <= num; c ++ ) {
if ( a ) {
dp[ a ][ b ][ c ][ 1 ] = min( dp[ a - 1 ][ b ][ c ][ 2 ], dp[ a - 1 ][ b ][ c ][ 3 ] );
mn = v[ a - 1 ].first;
mn -= a - 1;
mn -= f( v[ a - 1 ].second.first, b );
mn -= f( v[ a - 1 ].second.second, c );
dp[ a ][ b ][ c ][ 1 ] += mn;
}
if ( b ) {
dp[ a ][ b ][ c ][ 2 ] = min( dp[ a ][ b - 1 ][ c ][ 1 ], dp[ a ][ b - 1 ][ c ][ 3 ] );
mn = vv[ b - 1 ].first;
mn -= b - 1;
mn -= f( vv[ b - 1 ].second.first, a );
mn -= f( vv[ b - 1 ].second.second, c );
dp[ a ][ b ][ c ][ 2 ] += mn;
}
if ( c ) {
dp[ a ][ b ][ c ][ 3 ] = min( dp[ a ][ b ][ c - 1 ][ 1 ], dp[ a ][ b ][ c - 1 ][ 2 ] );
mn = v1[ c - 1 ].first;
mn -= c - 1;
mn -= f( v1[ c - 1 ].second.first, a );
mn -= f( v1[ c - 1 ].second.second, b );
dp[ a ][ b ][ c ][ 3 ] += mn;
}
// cout << a << " " << b << " " << c << " - " << dp[ a ][ b ][ c ][ 1 ] << " " << dp[ a ][ b ][ c ][ 2 ] << " " << dp[ a ][ b ][ c ][ 3 ] << "\n";
}
}
}
cout << min( dp[ sum ][ mx ][ num ][ 1 ], min( dp[ sum ][ mx ][ num ][ 2 ], dp[ sum ][ mx ][ num ][ 3 ] ) );
}
