Submission #6435712

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Source Code Expand

from collections import defaultdict


def main():
    N, M = map(int, input().split())
    X = list(map(int, input().split()))
    modcnt = [0] * M
    pairs = [0] * M
    cnt = defaultdict(lambda: 0)
    for x in X:
        cnt[x] += 1
        key = x % M
        modcnt[key] += 1
        if cnt[x] % 2 == 0:
            pairs[key] += 1
    ans = modcnt[0] // 2
    for i in range(1, M//2+1):
        if i * 2 == M:
            ans += modcnt[i] // 2
        else:
            tmp = min(modcnt[i], modcnt[M-i])
            ans += tmp
            ans += min((modcnt[i] - tmp) // 2, pairs[i])
            ans += min((modcnt[M-i] - tmp) // 2, pairs[M-i])
    print(ans)


if __name__ == "__main__":
    main()

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