Return the distance between <code>(x1, y1)</code> and <code>(x2, y2)</code>.
@param x1
the x component of the first vector
@param y1
the y component of the first vector
@param x2
the x component of the second vector
@param y2
the y component of the second vector
@return the euclidean distance
Return the distance between <code>(x1, y1)</code> and <code>(x2, y2)</code>.
@param x1 the x component of the first vector @param y1 the y component of the first vector @param x2 the x component of the second vector @param y2 the y component of the second vector @return the euclidean distance