function Main(input) {

	// 1行目がinput[0], 2行目がinput[1], …に入る
	input = input.split("\n");
    
    let [a,b,c,d] = input[0].split(" ").map(BigInt);
    
    let ac = a * c;
    let ad = a * d;
    let bc = b * c;
    let bd = b * d;
    
    let max = [ac,ad,bc,bd].sort((a, b) => (a < b) ? -1 : ((a > b) ? 1 : 0))[3];
    
    console.log(max.toString());
    
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));