#include <bits/stdc++.h>
#define fi first
#define se second
#define pb push_back
#define FOR(i, a, b) for (int i = a, _b = b; i <= _b; ++i)
#define FORD(i, a, b) for (int i = a, _b = b; i >= _b; --i)
#define FORLL(i, a, b) for (ll i = a, _b = b; i <= _b; ++i)
#define FORDLL(i, a, b) for (ll i = a, _b = b; i >= _b; --i)
#define all(x) x.begin(), x.end()
#define uni(x) sort(all(x)), x.erase(unique(all(x)), x.end())
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define dbg(...) debug(#__VA_ARGS__, __VA_ARGS__)
template<typename T>
void __print_one(const char *&s, const T &x)
{
while (*s == ' ') ++s;
const char *p = s;
int bal = 0;
while (*s)
{
if (*s == '(') ++bal;
else if (*s == ')') --bal;
else if (*s == ',' && bal == 0) break;
++s;
}
cerr.write(p, s - p) << " = " << x;
if (*s == ',')
{
++s;
cerr << " , ";
}
}
template<typename... Args>
void debug(const char *s, Args... args)
{
cerr << "[ ";
int dummy[] = { 0 , ( __print_one(s, args) , 0 )... };
(void)dummy;
cerr << " ]\n\n";
}
template<class X>
bool maximize(X &a, X b)
{
if (a < b)
{
a = b;
return true;
}
return false;
}
template<class X>
bool minimize(X &a, X b)
{
if (a > b)
{
a = b;
return true;
}
return false;
}
// --------------------------------------------------------------------------------------------
const int maxn = 405;
int n, f[maxn][maxn][3], dp[maxn][maxn][3], a[maxn], CNT[3][maxn], POS[3][maxn], sl[3];
// --------------------------------------------------------------------------------------------
void solve()
{
cin >> n;
FOR(i, 1, n)
{
char ch; cin >> ch;
if (ch == 'R') a[i] = 0;
else if (ch == 'G') a[i] = 1;
else a[i] = 2;
}
FOR(type, 0, 2)
FORD(i, n, 1)
CNT[type][i] = (a[i] == type) + CNT[type][i + 1];
FOR(type, 0, 2)
{
FOR(i, 1, n)
{
if (a[i] == type)
{
++sl[type];
POS[type][sl[type]] = i;
}
}
POS[type][sl[type] + 1] = n + 1;
}
FOR(lst, 0, 2)
dp[0][0][lst] = 0;
FOR(i, 1, n)
{
FOR(zero, 0, sl[0])
for (int one = 0; zero + one <= i; ++one)
FOR(lst, 0, 2)
{
f[zero][one][lst] = dp[zero][one][lst];
dp[zero][one][lst] = 1e9;
}
FOR(zero, 0, sl[0])
for (int one = 0; zero + one <= i && one <= sl[1]; ++one)
{
int two = i - zero - one;
if (two > sl[2]) continue;
FOR(lst, 0, 2)
{
if (lst == 0 && zero >= 1)
{
int pos = POS[0][zero] + max(0, CNT[1][POS[0][zero]] - CNT[1][POS[1][one] + 1])
+ max(0, CNT[2][POS[0][zero]] - CNT[2][POS[2][two] + 1]);
FOR(preLst, 0, 2)
{
if (lst == preLst) continue;
minimize(dp[zero][one][lst], f[zero - 1][one][preLst] + pos - i);
}
}
else if (lst == 1 && one >= 1)
{
int pos = POS[1][one] + max(0, CNT[0][POS[1][one]] - CNT[0][POS[0][zero] + 1])
+ max(0, CNT[2][POS[1][one]] - CNT[2][POS[2][two] + 1]);
FOR(preLst, 0, 2)
{
if (lst == preLst) continue;
minimize(dp[zero][one][lst], f[zero][one - 1][preLst] + pos - i);
}
}
else if (lst == 2 && two >= 1)
{
int pos = POS[2][two] + max(0, CNT[0][POS[2][two]] - CNT[0][POS[0][zero] + 1])
+ max(0, CNT[1][POS[2][two]] - CNT[1][POS[1][one] + 1]);
FOR(preLst, 0, 2)
{
if (lst == preLst) continue;
minimize(dp[zero][one][lst], f[zero][one][preLst] + pos - i);
}
}
}
}
}
int res = min({dp[sl[0]][sl[1]][0], dp[sl[0]][sl[1]][1], dp[sl[0]][sl[1]][2]});
cout << (res >= int(1e9) ? -1 : res);
}
signed main()
{
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
#define TASK "TEST"
if (fopen(TASK".INP", "r"))
{
freopen(TASK".INP", "r", stdin);
freopen(TASK".OUT", "w", stdout);
}
solve();
return 0;
}
