Correct answer: answer number 2 - -57/56

a = -1 =>
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b - 1 = c
c + d = e
e - 1 = f
b + d + f = -1/28

c = b - 1 =>
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b - 1 + d = e
e - 1 = f
b + d + f = -1/28


b + d = e + 1 =>
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e - 1 = f
e + 1 + f = -1/28

e = f + 1 =>
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f + 1 + 1 + f = -1/28 => 2 * f = -1/28 - 2 = -1/28 - 56/28 = -57/28 => f = -57/56