I am trying to Add two Numbers in a String and Return their summation , but facing an error in the hidden testcase

I was trying to answer this leetcode problem of Adding two Numbers in a String and returning a result. the code works fine for basic test cases, but in the hidden test cases the code fails to return a correct output and instead returns some unrelevent value. im having a hard time understanding that, could someone help?
the Input given in the hidden test case was :
Input
"3876620623801494171"
"6529364523802684779"
Output
"-8040758926105372666"
Expected
"10405985147604178950"

why is producing a negative value?

here is the code:
class Solution {
public String addStrings(String num1, String num2) {

       long a,b;
        a=Long.parseLong(num1);
        b=Long.parseLong(num2);
       a= a+b;
        
        num1=String.valueOf(a);
        
        return num1;
        
    }
}

>Solution :

It’s an integer overflow.

Your Value == 10405985147604178950

Long.MAX_VALUE == 9223372036854775807

You could use the BigInteger class to solve the problem.