((input) => {
  let [a,b,c,d] = input.split(' ').map(BigInt);
  let arr = [a*c,a*d,b*c,b*d];
  let max = arr.reduce((acc, curr) => {
    return acc > curr ? acc : curr;
  });

  console.log(max.toString());
})(require("fs").readFileSync("/dev/stdin", "utf8"));