#include <bits/stdc++.h>
using namespace std;
//#define FILE_IO
typedef pair<int, int> pii;
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<int> pos[3];
int cost(int x, int y)
{
if(x < y) return 0;
return (x - y);
}
int eval(string s1, string s2)
{
/*freopen("1.in", "r", stdin);
cin >> N;
string s1, s2;
cin >> s1 >> s2;*/
int ans = 0;
int f[3] = {0}, g[3] = {0};
for(int i = 0; i < s1.size(); i++)
{
int x = getid(s1[i]);
int y = getid(s2[i]);
f[x]++;
g[y]++;
if(g[y] > f[y])
{
for(int j = i + 1; j < s1.size(); j++)
if(getid(s1[j]) == y)
{
while(j > i)
{
swap(s1[j], s1[j - 1]);
j--;
ans++;
}
break;
}
}
f[x]--;
f[getid(s1[i])]++;
}
return ans;
//cout << s1 << '\n';
//cout << s2 << '\n';
//cout << ans << '\n';
//exit(0);
}
int main()
{
//eval();
#ifdef FILE_IO
freopen("1.in", "r", stdin);
freopen("1.out", "w", stdout);
#endif
cin >> N;
cin >> s;
for(int i = 0; i < N; i++)
pos[getid(s[i])].push_back(i + 1);
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], p);
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], p);
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], p);
lst[r][g][y][2] = (dp[r][g][y - 1][0] > dp[r][g][y - 1][1]) ? 1 : 0;
}
p++;
p--;
}
int ans = min({ dp[R][G][Y][0], dp[R][G][Y][1], dp[R][G][Y][2] });
int r, g, y, l;
r = R, g = G, y = Y;
if(ans == dp[r][g][y][0]) l = 0;
if(ans == dp[r][g][y][1]) l = 1;
if(ans == dp[r][g][y][2]) l = 2;
string mystr;
while(r + g + y > 0)
{
mystr.push_back(idtochar(l));
int ll = lst[r][g][y][l];
if(l == 0) r--;
if(l == 1) g--;
if(l == 2) y--;
l = ll;
}
reverse(mystr.begin(), mystr.end());
ans = eval(s, mystr);
cout << ans << '\n';
//cout << mystr << '\n';
return 0;
}
