#include <bits/stdc++.h>

using namespace std;

int main() {
    //cin读入优化
    std::ios::sync_with_stdio(false);

    double xa, ya, xb, yb, ab;
    cin >> xa >> ya >> xb >> yb;

    ab = sqrt((xa - xb) * (xa - xb) + (ya - yb) * (ya - yb));

    cout << fixed << setprecision(3) << ab << endl;
    return 0;
}