// parameter "input" gets all data
function Main(input) {
  input = input.split('\n')
  const chars = input[0].split(' ');
  const [a,b,c,d] = chars.map(x => parseInt(x,10));
  console.log(Math.max(a*c,a*d,b*c,b*d));
}
// Don't edit this line!
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
