#include <iostream>
using namespace std;
using ll = long long;
#define MIN(a, b) (((a) < (b)) ? (a) : (b))
#define MAX(a, b) (((a) < (b)) ? (b) : (a))
int const nmax = 401;
int dp[1 + nmax][1 + nmax][1 + nmax][4];
int v[1 + nmax];
int lower[1 + nmax][4];
int pos[1 + nmax][4];
bool worth(int a, int b, int c){
if(a + b + 1 < c)
return 0;
if(a + c + 1 < a)
return 0;
if(b + c + 1 < a)
return 0;
return 1;
}
int main()
{
int n;
cin >> n;
for(int i = 1;i <= n; i++) {
char ch;
cin >> ch;
if(ch == 'R')
v[i] = 1;
else if(ch == 'G')
v[i] = 2;
else if(ch == 'Y')
v[i] = 3;
lower[i][1] += lower[i - 1][1];
lower[i][2] += lower[i - 1][2];
lower[i][3] += lower[i - 1][3];
lower[i][v[i]]++;
pos[lower[i][v[i]]][v[i]] = i;
}
int red = lower[n][1], green = lower[n][2], yellow = lower[n][3];
for(int i = 0; i <= red; i++)
for(int j = 0; j <= green; j++)
for(int h = 0; h <= yellow; h++)
for(int last = 1; last <= 3; last++)
dp[i][j][h][last] = nmax * nmax;
dp[0][0][0][1] = dp[0][0][0][2] = dp[0][0][0][3] = 0;
for(int total = 0; total < n; total++)
for(int i = 0; i <= red; i++)
for(int j = 0; j <= green; j++)
if(i + j <= total){
int h = total - i - j;
if(worth(i, j, h) == 1){
if(i < red){
int pos2 = pos[i + 1][1];
dp[i + 1][j][h][1] = MIN(dp[i + 1][j][h][1], MIN(dp[i][j][h][2], dp[i][j][h][3]) +
MAX(0, lower[pos2][2] - j) +
MAX(0, lower[pos2][3] - h)
);
}
if(j < green){
int pos2 = pos[j + 1][2];
dp[i][j + 1][h][2] = MIN(dp[i][j + 1][h][2], MIN(dp[i][j][h][1], dp[i][j][h][3]) +
MAX(0, lower[pos2][1] - i) +
MAX(0, lower[pos2][3] - h)
);
}
if(h < yellow){
int pos2 = pos[h + 1][3];
dp[i][j][h + 1][3] = MIN(dp[i][j][h + 1][3], MIN(dp[i][j][h][1], dp[i][j][h][2]) +
MAX(0, lower[pos2][1] - i) +
MAX(0, lower[pos2][2] - j)
);
}
}
}
int result = MIN(dp[red][green][yellow][1], MIN(dp[red][green][yellow][2], dp[red][green][yellow][3]));
if(result == nmax * nmax)
cout << -1;
else
cout << result;
return 0;
}
