# include <iostream>
# include <cstring>
# include <string>
# include <vector>
# include <cmath>
# include <sstream>
# include <iomanip>
# include <algorithm>
using namespace std;

int main(){
	int  p[2], q[2];
	for(int i=0;i<2;i++){
		cin >> p[i];
	}
	for(int i=0;i<2;i++){
		cin >> q[i];
	}

	double data = abs(p[0] - q[0]) + abs(p[1] - q[1]);

	cout << fixed  << setprecision(1) << data/2.0 << endl;

}