public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

int N = sc.nextInt();

String s =sc.next();

char [] block = s.toCharArray();

int [] dp=new int [N];

Arrays.fill(dp,(int)1e9);

dp[0]=0;

int flag=1;

for(int i=0; i< block.length; i++){

if(block[i]=='B'){

for(int j=i; j< block.length; j++){

if(block[j]=='O'){

dp[j]=Math.min(dp[j],dp[i]+(j-i)*(j-i));

}

}

}

else if(block[i]=='O'){

for(int j=i; j< block.length; j++){

if(block[j]=='J'){

dp[j]=Math.min(dp[j],dp[i]+(j-i)*(j-i));

}

}

}

else if(block[i]=='J'){

for(int j=i; j< block.length; j++){

if(block[j]=='B'){

dp[j]=Math.min(dp[j],dp[i]+(j-i)*(j-i));

}

}

}

}

if(dp[N-1]==(int) 1e9){

System.out.println(-1);

}else{

System.out.println(dp[N-1]);

}

}

}