my code works on my ide but gives run time error on submission on codeforces.I can't understand why is this happening. Any help is appreciated.

here is my code:-

import java.util.*; public class leftHand_RightHand {

public static void main(String[] args) {
Scanner scn = new Scanner(System.in);

scn.nextLine();
String str=scn.nextLine();

// System.out.println(str+" "+str.length());

for(int i=0,j=(str.length()/2);i<(str.length()/2) && j<str.length();i++,j++) {
    if((str.charAt(i)=='L' && str.charAt(j)=='L') || ( str.charAt(i)=='R' &&  str.charAt(j)=='R'))
       System.out.println((i+1)+" "+(j+1));

    else if( str.charAt(i)=='L'&&  str.charAt(j)=='R')  
       System.out.println((i+1)+" "+(j+1));

    else if( str.charAt(i)=='R'&& str.charAt(j)=='L') 
       System.out.println((j+1)+" "+(i+1));
}

}

}