sum = (B - A - 1) * (A + B) / 2
例如：
A = -10, B = -5 
sum = (-5 - - 10 - 1) * (-10 + -5) / 2 = -30
A = -3, B = 5 
sum = (5 - -3 - 1) * (-3 + 5) / 2 = 7
A = 3, B = 5 
sum = (5 - 3 - 1) * (3 + 5) / 2 = 4
A = -5, B = -3 
sum = (-3 - -5 - 1) * (-5 + -3) / 2 = -4
A = 2, B = 6 
sum = (6 - 2 - 1) * (2 + 6) / 2 = 12
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