sum or sub two integers (without +/- operators)

/* Cpp program to sum or sub two integers (without +/- operators)
 * Author: Taqui ; Date : 14 July,2016
 * ###Copyright: All Rights Reserved###
 */

#include <iostream>
using namespace std;

// prototype declarations for sum and sub functions.
int summation(int,int);
int subtraction(int,int);

// main function starts:-
int main(void) {
    int a,b;
    cout << "Here a and b are two integers." << endl;
    cout << "Now,\ngive a: "; cin >> a;
    cout << "And,\ngive b: "; cin >> b;
    cout << "\nNow, Summation(a+b) of Subtraction(a-b)? Type (Sum/Sub): "; string choice; cin >> choice;
    if (choice=="Sum")
        cout << "\nThe summation(a+b) is: " << summation(a,b) << endl;
    else if (choice=="Sub")
        cout << "\nThe subtraction(a-b) is: " << subtraction(a,b) << endl;
    cout << "\nRe-run the program? Type (Yes/No): " << endl; cin >> choice;
    if (choice=="Yes")
        { cout << endl; main(); }
    else
        cout << "\nThank You." << endl;
    return 0;
}

// definitions of several declared functions:-
int summation (int x,int y) {
    if (y==0)
        return x;
    int carry= (x&y); // and operation
    x= x^y; // xor operation
    y= carry <<1; // left shift operator
    summation(x,y); // recursion
}
int subtraction (int x,int y) {
    if (y==0)
        return x;
    int borrow= (~x)&y;
    x= x^y;
    y= borrow <<1;
    subtraction(x,y);
}