Python Program:

Example:

n=int(input("Enter last number :"))
print("sum of 1 to ",n," : ", int((n*(n+1))/2))

Output:

  Enter last number : 10
  Sum of 1 to 10 : 55

We are using the formula of sum of natural numbers (1 to N)

(N*(N+1))/2  // N is the last number