import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
n = ii()
h = mii()
w = mii()
ans = 0
for i in range(n):
ans += (h[i] + h[i + 1]) * w[i]
p(ans / 2)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
solve()