#include <bits/stdc++.h>
#define task "testing"
#define multitest 0
using namespace std;
const int N = 400;
vector<int> pos[3];
int pre[3][N+5], dp[N+5][N+5][N+5][4], n, sz[3];
int calc(int x, int y, int z, int pc) {
if (x+y+z == n) return 0;
if (dp[x][y][z][pc] != -1) {
return dp[x][y][z][pc];
}
int res = 2303;
if (pc != 0 && x < sz[0]) {
int cur = pos[0][x], cst = 0;
cst += max(0, pre[1][cur]-y);
cst += max(0, pre[2][cur]-z);
res = min(res, calc(x+1, y, z, 0)+cst);
}
if (pc != 1 && y < sz[1]) {
int cur = pos[1][y], cst = 0;
cst += max(0, pre[0][cur]-x);
cst += max(0, pre[2][cur]-z);
res = min(res, calc(x, y+1, z, 1)+cst);
}
if (pc != 2 && z < sz[2]) {
int cur = pos[2][z], cst = 0;
cst += max(0, pre[0][cur]-x);
cst += max(0, pre[1][cur]-y);
res = min(res, calc(x, y, z+1, 2)+cst);
}
return dp[x][y][z][pc] = res;
}
void flo(int ID) {
cin >> n;
sz[0] = sz[1] = sz[2] = 0;
for (int x = 1; x <= n; x++) {
char ch; cin >> ch;
int v = 0;
if (ch == 'G') v = 1;
if (ch == 'Y') v = 2;
pos[v].push_back(x), sz[v]++;
for (int c = 0; c < 3; c++) {
pre[c][x] = pre[c][x-1];
}
pre[v][x]++;
}
int ans = calc(0, 0, 0, 3);
if (ans == 2303) ans = -1;
cout << ans << "\n";
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
if (fopen(task".inp", "r")) {
freopen(task".inp", "r", stdin);
freopen(task".out", "w", stdout);
}
memset(dp, -1, sizeof(dp));
int TCS = 1, ID = 1;
if (multitest) {
cin >> TCS;
}
while (TCS--) flo(ID++);
return 0;
}
