const main = arg => {
  let [a,b,c,d] = arg.trim().split("\n")[0].split(" ").map(e => BigInt(e))
  let max = a*c
  if (a*d > max) max = a*d
  if (b*c > max) max = b*c
  if (b*d > max) max = b*d
  console.log(max.toString())
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));