from typing import Counter


def sum_counters(counters):
    """
    Combines a bunch of counters.

    >>> c1 = Counter()
    >>> c1["a"] += 5
    >>> c2 = Counter()
    >>> c2["a"] += 1
    >>> c2["b"] -= 6
    >>> sum_counters([c1, c2])
    Counter({'a': 6, 'b': -6})
    """

    result = Counter()
    for counter in counters:
        result.update(counter)
    return result