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lqdojtocdovodoi
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a year ago
228 B
7
Indexable
n = int(input()) a = list(map(int, input().split())) dem = 0 cacso = {} for so in a: if -so in cacso: dem += cacso[-so] if so in cacso: cacso[so] += 1 else: cacso[so] = 1 print(dem)
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