How do you make an algebra function for this pattern?

The pattern goes like this: 1,3,6,10,15,21,28,etc.
Start with 1, add 2, add 3, etc.

I need a function that could find the value given the position in the pattern.
In this example an input of 4 would have an output of 10 since 10 is the value of the 4th position.

Any help on this would be greatly appreciated.

This is the function that you are looking for :

nth term = n*(n+1)/2

4th term = 4*(4+1)/2 = 10