#include<iostream>
#include<fstream>
#include<sstream>
#include<string>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<ctime>
#include<stack>
#include<queue>
#include<set>
#include<map>
#include<vector>
#include<list>
#include<algorithm>
#include<utility>
#include<complex>
#include<functional>
using namespace std;


#define LL long long
const long double eps = 1e-6;
int main(void){

	LL x, y, a, b;
	cin >> x >> y >> a >> b;
	bool flag = false;
	auto d = sqrt((long double)x*x + y*y) - sqrt((long double)(x - a)*(x - a) + (y - b)*(y - b)) - (sqrt((long double)a*a + b*b));
	if (d<eps&&d>-eps)flag = true;
	if (x == 0 || y == 0)flag = false;
	if (abs(x) != abs(y))flag = false;
	cout << max(abs(x), abs(y)) + flag << endl;

	return(0);
}