Adding BIG Integers In C++ an int is in the range 0 to 65535. But what if we need to add larger integers?Say we want compute the sum 2345566777844567+ 9999988777765768009998.You task in this assignment is to make this happen. Write a function string add(string a, string b)where a and b are strings representing integers and the function add(a, b) returns a string representing their sum. Strings a and b can have a maximum of 100 characters. Write a main program that loops asking the user for the two numbers a and b. The program should terminate if the user enters "done" what asked for "number" a. Code given by professor to start assigment:string add( string a, string b ) {string answer = "answer to be calcuated\n";return answer;}int main() {string x, y;for ( ;; ) {cout << "Enter two numbers a and b as string: " << "\t";cin >> x >> y;if ( x == "done" || y == "done" ){cout << "normal termination of the program" << endl;exit(1);}cout << x << " + " << y << " = " << add( x, y ) << endl;}return 0;}