what is time complexity of concatenating two int in java example

int a=18965;

int b=78521369741;

after concatenation i want ans in primitive integer data types like,

int c=1896578521369741;

i want to know what is the fastest way to do this and what will be the time complexity ?

private static void concat(long a, long b) {

long temp = b;
while (temp > 0) {
temp /= 10;
a *= 10;
}
a += b;
System.out.println(a);
}