def flatlandSpaceStations(n, m, c):
    if n == m:
        return 0
    c = sorted(c)
    start_distance = c[0]
    end_distance = (n-1) - c[-1]
    mid_distance = None
    for i in range(len(c)-1):
        gap = c[i+1] - c[i]
        distance = gap // 2
        if mid_distance is None or distance > mid_distance:
            mid_distance = distance
            
    return max(start_distance, mid_distance or 0, end_distance)