works/life/algorithm-experiment/5.1b.cpp


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#include <bitset>
#include <ios>
#include <iostream>

std::bitset<10000000> f;

int main() {
  std::ios_base::sync_with_stdio(false);
  std::cin.tie(nullptr);

  long long current;

  while (std::cin >> current) {
    f[current - 10000000].flip();
  }

  for (int i = 1; i < 10000000; ++i) {
    if (f[i]) {
      std::cout << i + 10000000;
      break;
    }
  }

  return 0;
}