Implement a program, which calculates the sum 1+2+3+...+n where n is given as user input.
Sample output:
Last number? **3**
The sum is 6
The previous example calculated 1 + 2 + 3 = 6
Last number? **7**
The sum is 28
And this one calculated 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28