var input = '';
var readable = process.stdin;
readable.resume();
readable.setEncoding( 'utf-8' );
readable.on( 'data', function( chunk ) {
  input += chunk;
});

readable.on( 'end', function() {
  var data = input.split( " " );
  var x1 = parseFloat( data[0] );
  var y1 = parseFloat( data[1] );
  var x2 = parseFloat( data[2] );
  var y2 = parseFloat( data[3] );
   
  var d = Math.sqrt( Math.pow( x1-x2, 2 )+Math.pow( y1-y2, 2 ) ).toFixed(5);
  console.log( d );
});