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( " " ).map( Number );
  var x1 = data[0];
  var y1 = data[1];
  var x2 = data[2];
  var y2 = data[3];
  
  var d = Math.sqrt( (x1-x2)*(x1-x2)+(y1-y2)*(y1-y2) );
  console.log( d );
});