using namespace std;
#include <iostream>
#include <cstring>
#define MAXN 405
int dp[MAXN][MAXN][MAXN][3];
int N,prefix[MAXN][3],pos[MAXN][3],num[3];
string S;
inline int getNum(char c) {
if(c == 'R') return 0;
if(c == 'G') return 1;
return 2;
}
int main() {
cin >> N >> S;
for(int i = 0;i < N;++i) {
for(int j = 0;j < 3;++j) {
prefix[i + 1][j] = prefix[i][j];
}
prefix[i + 1][getNum(S[i])]++;
pos[++num[getNum(S[i])]][getNum(S[i])] = i + 1;
}
// 2139062143
memset(dp,127,sizeof(dp));
for(int i = 0;i < 3;++i) {
dp[0][0][0][i] = 0;
}
for(int i = 0;i <= num[0];++i) {
for(int j = 0;j <= num[1];++j) {
for(int k = 0;k <= num[2];++k) {
for(int pre = 0;pre < 3;++pre) {
for(int next = 0;next < 3;++next) {
if(next == pre || (next == 0 && i == num[0]) || (next == 1 && j == num[1]) || (next == 2 && k == num[2])) continue;
int posA,posB,posC,other;
if(pre == 0) {
posA = pos[i][0];
if(next == 1) {
posB = pos[j + 1][1];
posC = pos[k][2];
other = 2;
}else{
posB = pos[k + 1][2];
posC = pos[j][1];
other = 1;
}
}else if(pre == 1) {
posA = pos[j][1];
if(next == 0) {
posB = pos[i + 1][0];
posC = pos[k][2];
other = 2;
}else{
posB = pos[k + 1][2];
posC = pos[i][0];
other = 0;
}
}else{
posA = pos[k][2];
if(next == 1) {
posB = pos[j + 1][1];
posC = pos[i][0];
other = 0;
}else{
posB = pos[i + 1][0];
posC = pos[j][1];
other = 1;
}
}
int total = max(0,prefix[posB][pre] - prefix[posA][pre]) + max(0,prefix[posB][other] - prefix[posC][other]) + dp[i][j][k][pre];
if(next == 0) {
dp[i + 1][j][k][next] = min(dp[i + 1][j][k][next],total);
}else if(next == 1) {
dp[i][j + 1][k][next] = min(dp[i][j + 1][k][next],total);
}else{
dp[i][j][k + 1][next] = min(dp[i][j][k + 1][next],total);
}
}
}
}
}
}
int ans = 2139062143;
for(int i = 0;i < 3;++i) {
ans = min(ans,dp[num[0]][num[1]][num[2]][i]);
}
cout << (ans == 2139062143 ? -1 : ans) << endl;
return 0;
}
