def maximumSum(a, m):
    prefixes = [(0, -1)]
    for i in range(len(a)):
        prefixes.append(((prefixes[-1][0] + a[i]) % m, i))
    prefixes.sort()
    maximum = prefixes[-1][0]
    for (a, i1), (b, i2) in zip(prefixes[:-1], prefixes[1:]):
        if b > a and i1 > i2:
            maximum = max(maximum, (a - b) % m)
    return maximum