#include <iostream>
#define f first
#define s second
using namespace std;
int n, a[410], dp[410][410][3], tmp[410][410][3], ps[3][410], b[3][410], cnt[3]; // R, G
string s;
int sum(int x, int y, int z, int id) {
pair<int,int> i, j, k;
int res = 0;
if (id == 0) i = {b[0][x],0}, j = min(make_pair(b[1][y],1),make_pair(b[2][z],2)), k = max(make_pair(b[1][y],1),make_pair(b[2][z],2));
else if (id == 1) i = {b[1][y],1}, j = min(make_pair(b[0][x],0),make_pair(b[2][z],2)), k = max(make_pair(b[0][x],0),make_pair(b[2][z],2));
else i = {b[2][z],2}, j = min(make_pair(b[0][x],0),make_pair(b[1][y],1)), k = max(make_pair(b[0][x],0),make_pair(b[1][y],1));
if (i > k) res += i.f-k.f-ps[i.s][i.f]+ps[i.s][k.f];
if (i > j) res += ps[j.s][min(i.f,k.f)]-ps[j.s][j.f];
return res;
}
int main() {
cin >> n >> s;
for (int i = 1; i <= n; i++) {
if (s[i-1] == 'R') a[i] = 0;
else if (s[i-1] == 'G') a[i] = 1;
else a[i] = 2;
b[a[i]][++cnt[a[i]]] = i;
for (int j = 0; j <= 2; j++) ps[j][i] = ps[j][i-1];
ps[a[i]][i]++;
}
for (int j = 0; j <= n; j++) for (int k = 0; k <= n; k++) for (int l = 0; l <= 2; l++) dp[j][k][l] = 1e9;
dp[0][0][0] = dp[0][0][1] = dp[0][0][2] = 0;
for (int i = 1; i <= n; i++) {
for (int j = 0; j <= n; j++) for (int k = 0; k <= n; k++) for (int l = 0; l <= 2; l++) tmp[j][k][l] = 1e9;
for (int x = 0; x <= cnt[0]; x++) {
for (int y = 0; y <= cnt[1]; y++) {
int z = i-x-y;
if ((x+y > i) || (z > cnt[2])) continue;
if (x) tmp[x][y][0] = min(dp[x-1][y][1]+sum(x,y,z,0),dp[x-1][y][2]+sum(x,y,z,0));
if (y) tmp[x][y][1] = min(dp[x][y-1][0]+sum(x,y,z,1),dp[x][y-1][2]+sum(x,y,z,1));
if (z) tmp[x][y][2] = min(dp[x][y][0]+sum(x,y,z,2),dp[x][y][1]+sum(x,y,z,2));
}
}
for (int j = 0; j <= n; j++) for (int k = 0; k <= n; k++) for (int l = 0; l <= 2; l++) dp[j][k][l] = tmp[j][k][l];
}
if (min(dp[cnt[0]][cnt[1]][0],min(dp[cnt[0]][cnt[1]][1],dp[cnt[0]][cnt[1]][2])) >= 1e9) cout << -1 << endl;
else cout << min(dp[cnt[0]][cnt[1]][0],min(dp[cnt[0]][cnt[1]][1],dp[cnt[0]][cnt[1]][2])) << endl;
}
