"use strict";
process.stdin.resume();
process.stdin.setEncoding('utf8');

var lines = []
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});

reader.on('line', (line) => {
  lines.push(line);
});

var re = /\s+|$/;
reader.on('close', () => {
    var f = lines[0].trim().split(' ').map(Number);
    console.log(aux(f[0],f[1],f[2],f[3]));
});

function aux(x0,y0,x1,y1)
{
    return Math.sqrt((x1-x0)*(x1-x0)+(y1-y0)*(y1-y0));
}
