#include<bits/stdc++.h>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) range(i,0,b)
#define debug(x) cout << "debug" << x << endl;
using namespace std;

int main(){
    double a, b, c, d;
    while(cin >> a >> b >> c >> d){
        double ans = sqrt( (c-a) * (c-a) + (d-b) * (d-b));
        printf("%.6lf",ans);
    }
}
#define _USE_MATH_DEFINES
#include <algorithm>
#include <cstdio>
#include <functional>
#include <iostream>
#include <cstring>
#include <climits>
#include <cmath>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <time.h>
#include <vector>
using namespace std;
 
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> i_i;
typedef pair<ll, int> ll_i;
typedef pair<double, int> d_i;
typedef pair<ll, ll> ll_ll;
typedef pair<double, double> d_d;
struct edge { int u, v; ll w; };
 
int MOD = 1000000007;

int main() {
	double x1, y1, x2, y2;
	cin >> x1 >> y1 >> x2 >> y2;
	double dx = x2 - x1, dy = y2 - y1;
	printf("%f\n", sqrt(dx * dx + dy * dy));
}