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import sys
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L = [list(map(int,l.split(' '))) for l in sys.stdin.read().splitlines()]
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def extrapolate(L):
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if all(not a for a in L):
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return [0]+L
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M = [b-a for a,b in zip(L,L[1:])]
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return [L[0]-extrapolate(M)[0]]+L
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print(sum(extrapolate(l)[0] for l in L))
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