#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int main() {
    
    double X[12] = { 100, 10, 1, 0.1, 0.01, 0.001, 0.0001, 0.00001, 0.000001, 0.0000001, 0.00000001, 0.000000001 };
    double xa,ya,xb,yb;
    cin >> xa >> ya >> xb >> yb;
    
    double ans = 0;
    
    for ( int i = 0; i < 12; i++ ) {
        
        double dist;
        double y;
        dist = 1000000;
        for ( int j = 0; j < 10; j++ ) {
            
            double y1 = ans + X[i]*j;
            double y2 = ans - X[i]*j;
            
            double dist1 = sqrt( xa*xa + (ya-y1)*(ya-y1) ) + sqrt( xb*xb + (yb-y1)*(yb-y1) );
            double dist2 = sqrt( xa*xa + (ya-y2)*(ya-y2) ) + sqrt( xb*xb + (yb-y2)*(yb-y2) );
            
            if ( dist > dist1 ) {
                dist = dist1;
                y = y1;
            }
            if ( dist > dist2 ) {                
                dist = dist2;
                y = y2;
            }
        }
        
        ans = y;
    }
    
    printf( "%.9f", ans );
    
    return 0;
}