C#: For the Loop 16.3 Help

Question

I need some guidance for the problem below. Been stuck on it for about an hour now.

The program you are given takes a positive number N as input.
Complete the program to calculate the sum of all numbers from 1 to N inclusive.

Sample Input
4

Sample Output
10

Answer 1

The sum from 1 to 4 is 1+2+3+4=10...
So use a for from 1 to n+1..

Answer 2

Or use a recursive method:

int Sum(int N){
if(N <= 1){
return 1;
}
return N + Sum(N - 1);
}

Answer 3

Or use the Gauss method n*(n+1)/2....with your example 4*(4+1)/2=10