def superDigit(n, k):
    sum1=sum(int(i) for i in  n)
    sum2=sum1*k
    while sum2>=10:
        sum2=sum(int(i) for i in str(sum2))
    return sum2

public static int superDigit(String n, int k) {
        if(n.length() == 1){
            return Integer.parseInt(n);
        }
        
        long sum = n.chars().mapToLong(x -> Integer.parseInt(Character.toString(x))).sum() * k;
        
        return superDigit(Long.toString(sum), 1);
    }

public static int superDigit(String n, int k) {
	    // Write your code here
	        BigInteger superDigit = new BigInteger(n);
	        StringBuilder sb = new StringBuilder();
	        for (int i = 0; i < k; i++) {
	            sb.append(n);
	        }
	        n = sb.toString();
	        char[] superDigitCharArray;
	        while(superDigit.compareTo(new BigInteger("10")) != -1){
	            int sum = 0;
	            superDigitCharArray = n.toCharArray();
	            for (char c : superDigitCharArray) {
	                sum += Character.getNumericValue(c); 
	            }
	            superDigit = new BigInteger(sum + "");
	            n = superDigit + "";
	        }
	        

	        return superDigit.intValue();
	    }