#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 xa,ya,xb,yb;
    cin >> xa >> ya >> xb >> yb;
    
    double y1,y2;
    y1 = ya; y2 = yb;
    if ( y1 > y2 ) { swap(y1, y2); }
    
    for ( int i = 0; i < 1000000; i++ ) {
        
        double y3,y4;
        y3 = y1 + (y2-y1)/3;
        y4 = y1 + ((y2-y1)/3)*2;
        
        double dist3 = sqrt( xa*xa + (ya-y3)*(ya-y3) ) + sqrt( xb*xb + (yb-y3)*(yb-y3) );
        double dist4 = sqrt( xa*xa + (ya-y4)*(ya-y4) ) + sqrt( xb*xb + (yb-y4)*(yb-y4) );
    
        if ( dist3 > dist4 ) {
            y1 = y3;
        }
        else {
            y2 = y4;
        }
    }
    
    printf( "%.7f", y1 );
    
    
    return 0;
}