process.stdin.resume();
process.stdin.setEncoding('utf8');
var lines = [];
var max;
var e = 1;
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});
reader.on('line', (line) => {
  lines.push(line);
});
reader.on('close', () => {
  var x = lines[0].split(' ');
  var a = parseInt(x[0]);
  var b= parseInt(x[1]);
  var c = parseInt(x[2]);
  var d = parseInt(x[3]);
  var y = b - a + 1;
  var u = d - c + 1;
  max = a * c;
  for(var i = 0;i<y;i++){
    for(var k = 0;k<u;k++){
      if(max < a * c){
        max = a * c;
      }
      c++;
    }
    c = parseInt(x[2]);
    a++;
  }
  if(max == 0){
    max = Math.abs(max);
  }
  console.log(max);
});