Python code for sum of n natural numbers:-

 #The sum of Natural numbers

a=int(input("from= "))
b=int(input("to = "))
sum = 0
for i in range(a,b+1):
    sum+= i

print("the sum is",sum)# Sum of natural numbers up to num

#sum of natural numbers using if, else and loop
num = int(input("Sum of natural number up to = "))

if num < 0:
   print()
else:
   sum = 0
   # use while loop to iterate until zero
   while(num > 0):
       sum += num
       num -= 1
   print("The sum is", sum)