#include<bits/stdc++.h>
using namespace std;
using ll = int;
const ll N = 402;
ll rg[N], ry[N], gr[N], gy[N], yr[N], yg[N];
ll dp[N][N][N][3];
vector < ll > red, green, yellow;
int main() {
ll n, m, r, x, y, i, j, cnt, s, R, G, Y, ans, t, g_cnt, y_cnt, r_cnt;
string str;
cin >> n >> str;
str = "." + str;
for ( R = 0; R <= n; R ++) for ( G = 0; G <= n; G ++) for ( Y = 0; Y <= n; Y ++ ) for ( r = 0; r <= 2; r ++) dp[R][G][Y][r] = 1e9;
for (i = 1; i <= n; i++) {
if ( str[i] == 'R') red.push_back(i);
if ( str[i] == 'G') green.push_back(i);
if ( str[i] == 'Y') yellow.push_back(i);
}
r_cnt = y_cnt = g_cnt = 0;
for (i = 0; i < red.size(); i ++) {
rg[i + 1] = lower_bound(green.begin(), green.end(), red[i]) - green.begin();
ry[i + 1] = lower_bound(yellow.begin(), yellow.end(), red[i]) - yellow.begin();
}
for (i = 0; i < green.size(); i ++) {
gr[i + 1] = lower_bound(red.begin(), red.end(), green[i]) - red.begin();
gy[i + 1] = lower_bound(yellow.begin(), yellow.end(), green[i]) - yellow.begin();
}
for (i = 0; i < yellow.size(); i ++) {
yg[i + 1] = lower_bound(green.begin(), green.end(), yellow[i]) - green.begin();
yr[i + 1] = lower_bound(red.begin(), red.end(), yellow[i]) - red.begin();
}
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for ( R = 0; R <= red.size(); R ++) {
for (G = 0; G <= green.size(); G ++) {
for ( Y = 0; Y <= yellow.size(); Y ++) {
// r = 0
// cout << R << " " << G << " " << Y << endl;
if ( R > 0 ) {
cnt = red[R - 1] - (R - 1);
cnt -= min(G, rg[R]);
cnt -= min(Y, ry[R]);
dp[R][G][Y][0] = min(dp[R][G][Y][0], dp[R - 1][G][Y][1] + cnt - 1);
dp[R][G][Y][0] = min(dp[R][G][Y][0], dp[R - 1][G][Y][2] + cnt - 1);
}
// r = 1
if ( G > 0 ) {
cnt = green[G - 1] - (G - 1);
cnt -= min(R, gr[G]);
cnt -= min(Y, gy[G]);
dp[R][G][Y][1] = min(dp[R][G][Y][1], dp[R][G - 1][Y][0] + cnt - 1);
dp[R][G][Y][1] = min(dp[R][G][Y][1], dp[R][G - 1][Y][2] + cnt - 1);
}
// r = 2;
if ( Y > 0) {
cnt = yellow[Y - 1] - (Y - 1);
cnt -= min(R, yr[Y]);
cnt -= min(G, yg[Y]);
dp[R][G][Y][2] = min(dp[R][G][Y][2], dp[R][G][Y - 1][0] + cnt - 1);
dp[R][G][Y][2] = min(dp[R][G][Y][2], dp[R][G][Y - 1][1] + cnt - 1);
}
}
}
}
// cout << "HI" << "\n"<< endl;
s = 1e9;
s = min(s, dp[red.size()][green.size()][yellow.size()][0]);
s = min(s, dp[red.size()][green.size()][yellow.size()][1]);
s = min(s, dp[red.size()][green.size()][yellow.size()][2]);
if ( s == 1e9) cout << -1 << endl;
else cout << s << endl;
}
