在线求解，有些疑惑，不理解！

i=1
sum=0
while i<=100:
    if i%2==0:
        sum+=i
        i+=1
    else:
        i+=1
print("1-100之间所有偶数的和为%d"%sum)

# 偶数之和
s = 0
for i in range(1,101):
   if i % 2 == 0:
      s += i
print(s)

#

start = 20
res = 1
while start >= 3:
   res *= start
   start -= 1
print(res)