Main/Python/2021/S1.py


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n = int(input())

heights = tuple(map(int, input().split()))

widths = tuple(map(int,input().split()))

base1 = heights[0]

area = 0
for i, w in enumerate(widths):
    area += (base1 + heights[i+1]) / 2 * w
    base1 = heights[i+1]

print(area)