手动模拟一下多项式乘法的操作就知道如何做了

from collections import defaultdict

a = {}
b = {}
product = defaultdict(float)

li = list(map(float,input().split()))
for i in range(1,len(li),2):
    a[int(li[i])] = li[i + 1]

li = list(map(float,input().split()))
for i in range(1,len(li),2):
    b[int(li[i])] = li[i + 1]

for k1,v1 in a.items():
    for k2,v2 in b.items():
        e = k1 + k2
        c = v1 * v2
        product[e] += c

cnt = 0
product = list(product.items())
product.sort(key = lambda item : -item[0])

for k,v in product:
    if v != 0:
        cnt += 1
print(cnt,end='')

for k,v in product:
    if v != 0:
        print(" %d %.1f" % (k,v),end = '')# 也可以使用格式化字符串的方式：print(f" {k} {v:.1f}", end='')或print(" {} {:.1f}".format(k, v), end='')