#include <iostream>
#include <cstring>
using namespace std;
constexpr int N = 400 + 5;
inline void minimize(int &dest, const int source) {
dest = min(dest, source);
}
int dp[2][N][N][3];
char s[N];
int cntR[N], cntG[N], cntY[N];
int posR[N], posG[N], posY[N];
signed main()
{
ios::sync_with_stdio(false);
cin.tie(NULL);
cout.tie(NULL);
// freopen("input.txt", "r", stdin);
int n;
cin >> n;
cin >> s;
cntR[0] = 0; for (int i = 0; i < n; i++) { cntR[i + 1] = cntR[i] + (s[i] == 'R'); }
cntG[0] = 0; for (int i = 0; i < n; i++) { cntG[i + 1] = cntG[i] + (s[i] == 'G'); }
cntY[0] = 0; for (int i = 0; i < n; i++) { cntY[i + 1] = cntY[i] + (s[i] == 'Y'); }
for (int i = 0; i <= n; i++) { posR[cntR[i]] = i; }
for (int i = 0; i <= n; i++) { posG[cntG[i]] = i; }
for (int i = 0; i <= n; i++) { posY[cntY[i]] = i; }
memset(dp[0], 0x3f, sizeof(dp[0]));
dp[0][0][0][0] = 0;
dp[0][0][0][1] = 0;
dp[0][0][0][2] = 0;
for (int i = 0; i < n; i++) {
for (int R = 0; R <= min(i + 1, cntR[n]); R++)
for (int G = 0; G <= min(i + 1 - R, cntG[n]); G++)
for (int last = 0; last < 3; last++)
dp[(i + 1) & 1][R][G][last] = 1e9;
// memset(dp[(i + 1) & 1], 0x3f, sizeof(dp[0]));
for (int R = 0; R <= min(i, cntR[n]); R++)
for (int G = 0; G <= min(i - R, cntG[n]); G++)
for (int last = 0; last < 3; last++)
{
int Y = i - R - G;
if (last != 0) {
int cG = max(0, cntG[posG[G]] - cntG[posR[R]]);
int cY = max(0, cntY[posY[Y]] - cntY[posR[R]]);
minimize(dp[(i + 1) & 1][R + 1][G][0], dp[i & 1][R][G][last] + cG + cY);
}
if (last != 1) {
int cR = max(0, cntR[posR[R]] - cntR[posG[G]]);
int cY = max(0, cntY[posY[Y]] - cntY[posG[G]]);
minimize(dp[(i + 1) & 1][R][G + 1][1], dp[i & 1][R][G][last] + cR + cY);
}
if (last != 2) {
int cR = max(0, cntR[posR[R]] - cntR[posY[Y]]);
int cG = max(0, cntG[posG[G]] - cntG[posY[Y]]);
minimize(dp[(i + 1) & 1][ R ][ G ][2], dp[i & 1][R][G][last] + cR + cG);
}
}
}
int res = min(dp[n & 1][cntR[n]][cntG[n]][0],
min(dp[n & 1][cntR[n]][cntG[n]][1],
dp[n & 1][cntR[n]][cntG[n]][2]));
cout << (res >= (int)1e9 ? -1 : res);
return 0;
}
