#include <bits/stdc++.h>
using namespace std;
//#define FILE_IO
typedef pair<int, int> pii;
typedef pair<int, pii> p3i;
int N;
string s;
int getid(char ch)
{
if(ch == 'R') return 0;
if(ch == 'G') return 1;
if(ch == 'Y') return 2;
return -1;
}
char idtochar(int id)
{
if(id == 0) return 'R';
if(id == 1) return 'G';
if(id == 2) return 'Y';
return 0;
}
int dp[405][405][405][3]; /// dp[r][g][y]
int lst[405][405][405][3];
vector<p3i> pos[3];
int cost(p3i x, p3i y)
{
int ans = 0;
ans += abs(x.first - y.first);
ans += abs(x.second.first - y.second.first);
ans += abs(x.second.second - y.second.second);
return ans;
}
int main()
{
#ifdef FILE_IO
freopen("1.in", "r", stdin);
freopen("1.out", "w", stdout);
#endif
cin >> N;
cin >> s;
int f[3];
f[0] = f[1] = f[2] = 0;
for(int i = 0; i < N; i++)
{
f[getid(s[i])]++;
pos[getid(s[i])].push_back({ f[0], {f[1], f[2]} });
}
int R = pos[0].size(), G = pos[1].size(), Y = pos[2].size();
int mx = max({R, G, Y});
if(mx > (N + 1) / 2)
{
printf("-1\n");
exit(0);
}
for(int r = 0; r <= R; r++)
for(int g = 0; g <= G; g++)
for(int y = 0; y <= Y; y++)
{
int p = r + g + y;
if(p == 0) continue;
dp[r][g][y][0] = dp[r][g][y][1] = dp[r][g][y][2] = 1 << 30;
if(r > 0)
{
dp[r][g][y][0] = min(dp[r - 1][g][y][1], dp[r - 1][g][y][2]) + cost(pos[0][r - 1], {r, {g, y}});
lst[r][g][y][0] = (dp[r - 1][g][y][1] > dp[r - 1][g][y][2]) ? 2 : 1;
}
if(g > 0)
{
dp[r][g][y][1] = min(dp[r][g - 1][y][0], dp[r][g - 1][y][2]) + cost(pos[1][g - 1], {r, {g, y}});
lst[r][g][y][1] = (dp[r][g - 1][y][0] > dp[r][g - 1][y][2]) ? 2 : 0;
}
if(y > 0)
{
dp[r][g][y][2] = min(dp[r][g][y - 1][0], dp[r][g][y - 1][1]) + cost(pos[2][y - 1], {r, {g, y}});
lst[r][g][y][2] = (dp[r][g][y - 1][0] > dp[r][g][y - 1][1]) ? 1 : 0;
}
}
int ans = min({ dp[R][G][Y][0], dp[R][G][Y][1], dp[R][G][Y][2] });
ans /= 2;
cout << ans << '\n';
return 0;
}
