// oooo
/*
har chi delet mikhad bebar ~
gitar o ba khodet nabar! ~
;Amoo_Hasan;
*/
#include<bits/stdc++.h>
//#pragma GCC optimize("O3,no-stack-protector,unroll-loops")
//#pragma GCC target("avx2,fma")
using namespace std;
typedef long long ll;
typedef long double ld;
#define Sz(x) int((x).size())
#define All(x) (x).begin(), (x).end()
#define wtf(x) cout<<#x <<" : " <<x
constexpr int N = 410, inf = 510510;
int dp[N][N][N][3], ind[3][N], tim[3], pr[N][3];
int main() {
ios :: sync_with_stdio(0), cin.tie(0);
int n; cin >>n;
string s; cin >>s;
s = '#' + s;
for(int i = 1; i <= n; i++) {
if(s[i] == 'R') s[i] = 'a';
else if(s[i] == 'G') s[i] = 'b';
else s[i] = 'c';
}
for(int i = 1; i <= n; i++) {
ind[s[i] - 'a'][++tim[s[i] - 'a']] = i;
}
int cnt0 = 0, cnt1 = 0, cnt2 = 0;
for(int i = 1; i <= n; i++) {
if(s[i] - 'a' == 0) cnt0++;
else if(s[i] - 'a' == 1) cnt1++;
else cnt2++;
}
for(int i = 1; i <= n; i++) {
for(int j = 0; j <= 2; j++) pr[i][j] = pr[i - 1][j];
pr[i][s[i] - 'a']++;
}
for(int i = 0; i <= cnt0; i++) {
for(int j = 0; j <= cnt1; j++) {
for(int k = 0; k <= cnt2; k++) {
for(int f = 0; f <= 2; f++) {
dp[i][j][k][f] = inf;
}
}
}
}
dp[0][0][0][0] = dp[0][0][0][1] = dp[0][0][0][2] = 0;
int ans;
for(int i = 0; i <= cnt0; i++)
for(int j = 0; j <= cnt1; j++)
for(int k = 0; k <= cnt2; k++)
for(int l = 0; l <= 2; l++)
for(int cur = 0; cur <= 2; cur++) {
if(cur == l) continue;
if(cur == 0 && cnt0 == i) continue;
if(cur == 1 && cnt1 == j) continue;
if(cur == 2 && cnt2 == k) continue;
int pos1 = 0, pos2 = 0, pos3 = 0;
int res = 3 - l - cur;
if(l == 0) {
pos1 = ind[0][i];
if(cur == 1) pos2 = ind[1][j + 1], pos3 = ind[2][k];
else pos2 = ind[2][k + 1], pos3 = ind[1][j];
}
else if(l == 1) {
pos1 = ind[1][j];
if(cur == 0) pos2 = ind[0][i + 1], pos3 = ind[2][k];
else pos2 = ind[2][k + 1], pos3 = ind[0][i];
}
else if(l == 2) {
pos1 = ind[2][k];
if(cur == 0) pos2 = ind[0][i + 1], pos3 = ind[1][j];
else pos2 = ind[1][j + 1], pos3 = ind[0][i];
}
ans = 0;
ans += max(0, pr[pos2][l] - pr[pos1][l]);
ans += max(0, pr[pos2][res] - pr[pos3][res]);
ans += dp[i][j][k][l];
if(cur == 0) {
dp[i + 1][j][k][cur] = min(dp[i + 1][j][k][cur], ans);
}
else if(cur == 1) {
dp[i][j + 1][k][cur] = min(dp[i][j + 1][k][cur], ans);
}
else {
dp[i][j][k + 1][cur] = min(dp[i][j][k + 1][cur], ans);
}
}
ans = inf;
for(int i = 0; i < 3; i++) ans = min(ans, dp[cnt0][cnt1][cnt2][i]);
if(ans >= inf) cout<<-1;
else cout<<ans;
return 0;
}
