((input) => {
  let [a,b,c,d] = input.trim().split(' ').map(Number);
  let arr = [a*c,a*d,b*c,b*d];
  let res = arr.reduce((a, b) => Math.max(a, b));
  console.log(res);
})(require("fs").readFileSync("/dev/stdin", "utf8"));