import math


def distance(a, b):
    """Calculates distance between two latitude-longitude coordinates."""
    R = 3963  # radius of Earth (miles)
    lat1, lon1 = math.radians(a[0]), math.radians(a[1])
    lat2, lon2 = math.radians(b[0]), math.radians(b[1])
    return math.acos(math.sin(lat1) * math.sin(lat2) +
                     math.cos(lat1) * math.cos(lat2) * math.cos(lon1 - lon2)) * R