//config = {stdin: 'tmp', newline: '\r\n'}; // win
config = {stdin: '/dev/stdin', newline: '\n'}; // linux

line = require('fs').readFileSync(config.stdin, 'ascii').split(' ')
       .map(Number);
dx = line[2] - line[0], dy = line[3] - line[1];
console.log(Math.sqrt(dx * dx + dy * dy));