'use strict'

const main = arg => {
    var input = arg.trim().split("\n")[0].split(" ");
    const x1 = input[0];
    const y1 = input[1];
    const x2 = input[2];
    const y2 = input[3];
    const result = Math.sqrt( Math.pow(x2 - x1, 2) + Math.pow(y2 - y1, 2));

    console.log(`${result.toFixed(8)}`);
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));
