#include <iostream>
#include <cmath>
#include <complex>

using namespace std;

typedef complex<double> vect;

double dot(vect &a, vect &b)
{
    return real(conj(a)*b);
}

int main()
{
    double a1,a2,b1,b2;
    cin >> a1 >> a2 >> b1 >> b2;

    vect a(a1,a2), b(b1,b2);

    //cout << dot(a,b) << '\n';

    double ang = acos( dot(a,b) / (sqrt(real(a)*real(a)+imag(a)*imag(a))*sqrt(real(b)*real(b)+imag(b)*imag(b))) );
    cout.precision(5);

    cout << fixed << ang << '\n';
    return 0;
}