#define DEBUG 0
#include <bits/stdc++.h>
using namespace std;
#if DEBUG
// basic debugging macros
int __i__,__j__;
#define printLine(l) for(__i__=0;__i__<l;__i__++){cout<<"-";}cout<<endl
#define printLine2(l,c) for(__i__=0;__i__<l;__i__++){cout<<c;}cout<<endl
#define printVar(n) cout<<#n<<": "<<n<<endl
#define printArr(a,l) cout<<#a<<": ";for(__i__=0;__i__<l;__i__++){cout<<a[__i__]<<" ";}cout<<endl
#define print2dArr(a,r,c) cout<<#a<<":\n";for(__i__=0;__i__<r;__i__++){for(__j__=0;__j__<c;__j__++){cout<<a[__i__][__j__]<<" ";}cout<<endl;}
#define print2dArr2(a,r,c,l) cout<<#a<<":\n";for(__i__=0;__i__<r;__i__++){for(__j__=0;__j__<c;__j__++){cout<<setw(l)<<setfill(' ')<<a[__i__][__j__]<<" ";}cout<<endl;}
// advanced debugging class
// debug 1,2,'A',"test";
class _Debug {
public:
template<typename T>
_Debug& operator,(T val) {
cout << val << endl;
return *this;
}
};
#define debug _Debug(),
#else
#define printLine(l)
#define printLine2(l,c)
#define printVar(n)
#define printArr(a,l)
#define print2dArr(a,r,c)
#define print2dArr2(a,r,c,l)
#define debug
#endif
// define
#define MAX_VAL 999999999
#define MAX_VAL_2 999999999999999999LL
#define EPS 1e-6
#define mp make_pair
#define pb push_back
// typedef
typedef unsigned int UI;
typedef long long int LLI;
typedef unsigned long long int ULLI;
typedef unsigned short int US;
typedef pair<int,int> pii;
typedef pair<LLI,LLI> plli;
typedef vector<int> vi;
typedef vector<LLI> vlli;
typedef vector<pii> vpii;
typedef vector<plli> vplli;
// ---------- END OF TEMPLATE ----------
char S[400];
vi c[3];
int num[3][401][400];
int dp[2500000][3];
int main() {
int i;
int N;
scanf("%d",&N);
for (i = 0; i < N; i++) {
scanf(" %c",&S[i]);
if (S[i] == 'R') c[0].pb(i);
else if (S[i] == 'G') c[1].pb(i);
else c[2].pb(i);
}
int j,k;
int x = c[0].size()+1,y = c[1].size()+1,z = c[2].size()+1;
for (i = 0; i < 3; i++) {
for (j = 0; j < c[i].size(); j++) {
for (k = 0; k < N; k++) num[i][j+1][k] = num[i][j][k];
for (k = 0; k <= c[i][j]; k++) num[i][j+1][k]++;
}
}
for (i = 1; i < x*y*z; i++) {
int p = i % x,q = (i/x) % y,r = i/(x*y);
for (j = 0; j < 3; j++) dp[i][j] = 1e9;
if (p > 0) dp[i][0] = min(dp[i][0],min(dp[i-1][1],dp[i-1][2])+num[1][q][c[0][p-1]]+num[2][r][c[0][p-1]]);
if (q > 0) dp[i][1] = min(dp[i][1],min(dp[i-x][0],dp[i-x][2])+num[0][p][c[1][q-1]]+num[2][r][c[1][q-1]]);
if (r > 0) dp[i][2] = min(dp[i][2],min(dp[i-x*y][0],dp[i-x*y][1])+num[0][p][c[2][r-1]]+num[1][q][c[2][r-1]]);
}
i--;
int ans = min(min(dp[i][0],dp[i][1]),dp[i][2]);
printf("%d\n",(ans < 1e8) ? ans:-1);
return 0;
}
