this is my solution

public static void main(String[] args) { Scanner in = new Scanner(System.in); String s = in.next(); long n = in.nextLong();

    long numOfChar = 0 ;
    int numOfA = 0;
    int stringLength = s.length();

    if(stringLength > 0){
       for(int i=0;i<stringLength;i++){
          if(s.charAt(i)=='a'){
            numOfA++;
          } 
       } 

       long division = n / stringLength;
       long mod = n % stringLength;
       numOfChar = division * numOfA;

       for(int i=0;i<mod;i++){
           if(s.charAt(i)=='a'){
               numOfChar++;
           }
       }

    }

    System.out.println(numOfChar);
}