const main = arg => {
  const [a, b, c, d] = arg.split(' ').map(BigInt);
  const ary = [a * c, a * d, b * c, b * d];
  const max = ary.reduce((x, y) => x > y ? x : y);
  console.log(max.toString());
};
main(require('fs').readFileSync('/dev/stdin', 'utf8'));