'use strict';

const main = arg => {
  const [a, b, c, d] = arg.trim().split(' ').map(x => BigInt(x));
  var ans = a * c;
  if (ans < a * d) ans = a * d;
  if (ans < b * c) ans = b * c;
  if (ans < b * d) ans = b * d;
  console.log(ans.toString());
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));