#include <bits/stdc++.h>
#define y1 uidi

using namespace std;

int x1, y1;
int x2, y2;

double angle(int x1, int y1, int x2, int y2) {
    double t = (x1 * x2 + y1 * y2) /
               (sqrt((double)x1 * x1 + y1 * y1) *
                sqrt((double)x2 * x2 + y2 * y2));
    if      (t < -1) t = -1;
    else if (t > 1)  t = 1;
    return acos(t);
}

int main() {
    scanf("%d %d %d %d", &x1, &y1, &x2, &y2);

    printf("%f", angle(x1, y1, x2, y2));

    return 0;
}