Competitions

The angle between the vectors

Calculate the angle between two vectors.

Input

Four integers x1,y1,x2,y2 - the coordinates of two nonzero vectors. All values do not exceed 10000 by the absolute value.

Output

Print the value of an undirected angle between vectors with 5 decimal places in the interval [0; π].

Time limit 1 second
Memory limit 122.17 MiB
Input example #1
2 1 3 5

Output example #1
0.56673