#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define all(a) begin(a),end(a)
const int N = (int)4e2+2;
const int INF = (int)1e9;
vector<int> v[3];
int dp[N][3][N/2][N];
int n, cnt[3], chk[3][3][N];
int num(int cnt0, int cnt1, int cnt2, int f0, int f1, int f2){
return max(0,cnt1-chk[f0][f1][cnt0])+max(0,cnt2-chk[f0][f2][cnt0]);
}
void swp(int i, int j){ swap(cnt[i],cnt[j]); swap(v[i],v[j]); }
int main(){
ios_base::sync_with_stdio(false); cin.tie(0);
string s; cin >> n >> s;
for(int i = 0; i < n; i++){
char u = s[i];
if(u=='R') s[i]=0;
else if(u=='G') s[i]=1;
else s[i]=2;
cnt[s[i]]++; v[s[i]].pb(i+1);
}
if(cnt[0]>cnt[1])swp(0,1);
if(cnt[1]>cnt[2])swp(1,2);
if(cnt[0]>cnt[1])swp(0,1);
memset(dp,0x3f3f3f3f,sizeof(dp));
for(int i : {0,1,2}) dp[0][i][0][0]=0;
for(int f0 = 0; f0 < 3; f0++)
for(int f1 = 0; f1 < 3; f1++)
for(int cnt0 = 1; cnt0 <= cnt[f0] and f0!=f1; cnt0++)
chk[f0][f1][cnt0] = lower_bound(all(v[f1]),v[f0][cnt0-1])-begin(v[f1]);
int ans = INF;
for(int i = 1; i <= n; i++){
for(int j = 0; j < 3; j++){
for(int m = 0; m < 3; m++){
for(int k = 0; k <= min(i,cnt[0]) and m!=j; k++){
for(int l = 0; l <= min(i,cnt[1]); l++){
int o = i-k-l; if(o>i or o>cnt[2]) continue;
if(k and j==0){
int f0 = v[0][k-1]+num(k,l,o,0,1,2);
if(f0>=i) dp[i][j][k][l] = min(dp[i][j][k][l], dp[i-1][m][k-1][l]+f0-i);
}
if(l and j==1){
int f1 = v[1][l-1]+num(l,k,o,1,0,2);
if(f1>=i) dp[i][j][k][l] = min(dp[i][j][k][l], dp[i-1][m][k][l-1]+f1-i);
}
if(o and j==2){
int f2 = v[2][o-1]+num(o,k,l,2,0,1);
if(f2>=i) dp[i][j][k][l] = min(dp[i][j][k][l], dp[i-1][m][k][l]+f2-i);
}
}
}
}
}
}
for(int j = 0; j < 3; j++)
for(int k = 0; k <= cnt[0]; k++)
for(int l = 0; l <= cnt[1]; l++)
ans = min(ans, dp[n][j][k][l]);
if(ans>=INF) ans=-1;
cout << ans << "\n";
}
