3099. Harshad Number 

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class Solution {
    public int sumOfTheDigitsOfHarshadNumber(int x) {
        int sum = 0, tmp = x;
        while (tmp > 0) {
            sum += tmp % 10;
            tmp /= 10;
        }

        if (x % sum == 0) {
            return sum;
        } else {
            return -1;
        }
    }
}

Reference

3099. Harshad Number