#include<algorithm>
#include<complex>
#include<iostream>
#include<map>
#include<queue>
#include<set>
#include<stdint.h>
#include<vector>
#ifdef LOCAL
#include "dbg.h"
#else
#define dbg(...)
#endif
using namespace std;
const int64_t INF = (int64_t) 1e18 + 777;
const int64_t mINF = (int64_t) 1e9 + 777;
const int64_t MOD = (int64_t) 1e9 + 7;
struct Solution
{
Solution() {}
void solve()
{
int n;
string s;
cin >> n >> s;
int k = 3;
vector<vector<int> > colors(k);
// ???
for (int i = 0; i < k; i++) {
colors.reserve(n);
}
vector<vector<int> > prefix(k, vector<int>(n + 1, 0));
for(int i = 0; i < n; i++)
{
int x = getID(s[i]);
colors[x].push_back(i);
prefix[x][i + 1] = 1;
}
for(int i = 0; i < k; i++)
{
colors[i].push_back(n);
// sort(colors[i].begin(), colors[i].end());
for(int j = 1; j <= n; j++)
{
prefix[i][j] += prefix[i][j - 1];
}
}
int r = colors[0].size();
int g = colors[1].size();
int y = colors[2].size();
vector<vector<vector<vector<int> > > > dp(2, vector<vector<vector<int > > >(k, vector<vector<int> >(r, vector<int>(g, mINF))));
if(r > 1) dp[0][0][1][0] = colors[0][0];
if(g > 1) dp[0][1][0][1] = colors[1][0];
if(y > 1) dp[0][2][0][0] = colors[2][0];
int t = 1;
for(int i = 1; i < n; i++)
{
int p = t ^ 1;
for(int last = 0; last < k; last++)
{
for(int cr = 0; cr < r; cr++)
{
for(int cg = 0; cg < g; cg++)
{
if(dp[p][last][cr][cg] == mINF) continue;
int cy = i - cr - cg;
// red
// A = last B = now C = other
if(last != 0 && cr != r - 1)
{
int a = last;
int b = 0;
int c = 0;
int cur_a = 0;
int cur_c = 0;
int cur_b = colors[b][cr];
if(a == 1)
{
c = 2;
cur_a = colors[a][cg];
cur_c = colors[c][cy];
}
else
{
c = 1;
cur_a = colors[a][cy];
cur_c = colors[c][cg];
}
int cnt1 = prefix[a][cur_b + 1] - prefix[a][cur_a];
int cnt2 = prefix[c][cur_b + 1] - prefix[c][cur_c];
cnt1 = max(cnt1, 0); cnt2 = max(cnt2, 0);
int cost = cnt1 + cnt2;
dp[t][b][cr + 1][cg] = min(dp[t][b][cr + 1][cg], dp[p][last][cr][cg] + cost);
}
// green
if(last != 1 && cg != g - 1)
{
int a = last;
int b = 1;
int c = 0;
int cur_a = 0;
int cur_c = 0;
int cur_b = colors[b][cg];
if(a == 0)
{
c = 2;
cur_a = colors[a][cr];
cur_c = colors[c][cy];
}
else
{
c = 0;
cur_a = colors[a][cy];
cur_c = colors[c][cr];
}
int cnt1 = prefix[a][cur_b + 1] - prefix[a][cur_a];
int cnt2 = prefix[c][cur_b + 1] - prefix[c][cur_c];
cnt1 = max(cnt1, 0); cnt2 = max(cnt2, 0);
int cost = cnt1 + cnt2;
dp[t][b][cr][cg + 1] = min(dp[t][b][cr][cg + 1], dp[p][last][cr][cg] + cost);
}
// yellow
if(last != 2 && cy != y - 1)
{
int a = last;
int b = 2;
int c = 0;
int cur_a = 0;
int cur_c = 0;
int cur_b = colors[b][cy];
if(a == 0)
{
c = 1;
cur_a = colors[a][cr];
cur_c = colors[c][cg];
}
else
{
c = 0;
cur_a = colors[a][cg];
cur_c = colors[c][cr];
}
int cnt1 = prefix[a][cur_b + 1] - prefix[a][cur_a];
int cnt2 = prefix[c][cur_b + 1] - prefix[c][cur_c];
cnt1 = max(cnt1, 0); cnt2 = max(cnt2, 0);
int cost = cnt1 + cnt2;
dp[t][b][cr][cg] = min(dp[t][b][cr][cg], dp[p][last][cr][cg] + cost);
}
}
}
}
for(int last = 0; last < k; last++)
{
for(int cr = 0; cr < r; cr++)
{
for(int cg = 0; cg < g; cg++)
{
dp[p][last][cr][cg] = mINF;
}
}
}
t ^= 1;
}
int ans = mINF;
for(int last = 0; last < k; last++)
{
ans = min(ans, dp[t ^ 1][last][r - 1][g - 1]);
}
if(ans == mINF) ans = -1;
cout << ans << "\n";
}
int getID(char c)
{
if(c == 'R') return 0;
if(c == 'G') return 1;
if(c == 'Y') return 2;
return -1;
}
int modadd(int a, int b)
{
int res = a + b;
if(res >= MOD) res -= MOD;
return res;
}
int modmul(int a, int b)
{
return (1LL * a * b) % MOD;
}
};
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int t = 1;
//cin >> t;
for(int i = 0; i < t; i++)
{
Solution().solve();
}
return 0;
}
