#include <iostream>
#include <cmath>
#include <iostream>
#include <iterator>
#include <map>
#include <set>
#include <string>
#include <vector>
#include <algorithm>
#include <stack>
#include <deque>
#include <queue>
#include <climits>

#define int long long
#define db double
#define ff first
#define ss second

using namespace std;

const int MOD = 1e9 + 7;
// const int INF = LONG_MAX;
const int INF = INT_MAX;
const int MAXW = 10001;
const double PI = 3.14159265358979323846;

struct Point {
    int x;
    int y;

    Point& operator += (const Point& p) {
        x += p.x;
        y += p.y;
        return *this;
    }

    Point& operator -= (const Point& p) {
        x -= p.x;
        y -= p.y;
        return *this;
    }
};

ostream& operator << (ostream& out, Point& p) {
    out << p.x << " " << p.y << '\n';
    return out;
}

istream& operator >> (istream& in, Point& p) {
    in >> p.x >> p.y;
    return in;
}

struct Vct {
    int x;
    int y;

    Vct& operator += (const Vct& a) {
        x += a.x;
        y += a.y;
        return *this;
    }

    Vct& operator -= (const Vct& a) {
        x -= a.x;
        y -= a.y;
        return *this;
    }

    Vct& operator *= (const int c) {
        x *= c;
        y *= c;
        return *this;
    }
};

Vct operator + (const Vct& a, const Vct& b) {
    Vct tmp = a;
    tmp += b;
    return tmp;
}

Vct operator - (const Vct& a, const Vct& b) {
    Vct tmp = a;
    tmp -= b;
    return tmp;
}

ostream& operator << (ostream& out, Vct& v) {
    out << v.x << " " << v.y << '\n';
    return out;
}

istream& operator >> (istream& in, Vct& v) {
    in >> v.x >> v.y;
    return in;
}

signed main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    Vct f, s;
    cin >> f >> s;
    double ans = abs(atan2(f.y, f.x) - atan2(s.y, s.x));
    if (ans > PI) {
        ans -= PI;
    }
    if (abs(2 * PI - ans) < ans) {
        ans = abs(2 * PI - ans);
    }
    cout.precision(20);
    cout << fixed << ans;
}