2's complement in C++

#include <iostream>

#define size 6
using namespace std;

int main() {
   char binary[size + 1], one[size + 1], two[size + 1];
   int i, carry = 1, fail = 0;
   cout << "Input a " << size << " bit binary number: ";
   cin >> binary;
   for (i = 0; i < size; i++) {
      if (binary[i] == '1') {
         one[i] = '0';
      } else if (binary[i] == '0') {
         one[i] = '1';
      } else {
         cout << "Error! Input the number of assigned bits." << endl;
         fail = 1;
         break;
      }
   }
   one[size] = '\0';

   for (i = size - 1; i >= 0; i--) {
      if (one[i] == '1' && carry == 1) {
         two[i] = '0';
      } else if (one[i] == '0' && carry == 1) {
         two[i] = '1';
         carry = 0;
      } else {
         two[i] = one[i];
      }
   }
   two[size] = '\0';
   if (fail == 0) {
      cout << "The original binary = " << binary << endl;
      cout << "After ones complement the value = " << one << endl;
      cout << "After twos complement the value = " << two << endl;
   }
}

Output

Input a 6 bit binary number: 101101
The original binary = 101101
After ones complement the value = 010010
After twos complement the value = 010011