def flatlandSpaceStations(n, c, m):
    c.sort()
    maxi=max(abs(c[0]-0),abs((n-1)-c[-1]))
    if n<=m:
        return '0'
    for i in range(m-1):
        maxi=max(abs(c[i]-c[i+1])//2,maxi)
    return maxi