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 d = Math.sqrt( Math.pow( data[0]-data[2], 2 )+Math.pow( data[1]-data[3], 2 ) ).toFixed(5);
  console.log( d );
});