#include <bits/stdc++.h>
using namespace std;
const int INF = 1e9;
int dp[3][401][401][401],pos[3][401],cnt[3];
int cost[3][3][401][401];
void chmin(int &x,int y){
x = min(x,y);
}
int main()
{
int n;
string s;
cin>>n>>s;
vector<int>c;
for(char x:s){
if(x == 'R')c.push_back(0);
if(x == 'G')c.push_back(1);
if(x == 'Y')c.push_back(2);
}
for(int i=0;i<=n;i++)pos[0][i] = pos[1][i] = pos[2][i] = -1;
for(int i=0;i<n;i++)pos[c[i]][cnt[c[i]]++] = i;
/*
for(int i=0;i<3;i++){
for(int j=0;j<cnt[i];j++)cout<<pos[i][j]<<" ";
cout<<'\n';
}*/
for(int c1=0;c1<3;c1++){
for(int c2=0;c2<3;c2++){
if(c1 == c2)continue;
for(int i=0;i<cnt[c1];i++){
for(int j=0;j<cnt[c2];j++){
cost[c1][c2][i][j] = pos[c2][j] > pos[c1][i];
if(j)cost[c1][c2][i][j]+=cost[c1][c2][i][j-1];
}
}
}
}
for(int i=0;i<=n;i++)
for(int j=0;j<=i;j++)
for(int k=0;k+j<=i;k++)
dp[0][i][j][k] = dp[1][i][j][k] = dp[2][i][j][k] = INF;
dp[0][0][0][0] = dp[1][0][0][0] = dp[2][0][0][0] = 0;
array<int,3>p;
for(int i=0;i<n;i++){
for(int j=0;j<=i;j++){
for(int k=0;k+j<=i;k++){
p = {j,k,i-j-k}; //number of rgy
for(int c1=0;c1<3;c1++){ //previous color
if(dp[c1][i][j][k] == INF)continue;
for(int c2=0;c2<3;c2++){ //current color
if(c1 == c2 || pos[c2][p[c2]] == -1)continue;
int tot = 0;
for(int c3=0;c3<3;c3++){
if(p[c3])tot+=cost[c2][c3][p[c2]][p[c3]-1];
}
if(c2 == 0)chmin(dp[0][i+1][j+1][k],dp[c1][i][j][k] + tot);
if(c2 == 1)chmin(dp[1][i+1][j][k+1],dp[c1][i][j][k] + tot);
if(c2 == 2)chmin(dp[2][i+1][j][k],dp[c1][i][j][k] + tot);
}
}
}
}
}
int mn = INF;
for(int i=0;i<3;i++)mn = min(mn,dp[i][n][cnt[0]][cnt[1]]);
if(mn == INF)mn = -1;
cout<<mn<<'\n';
}
