Nth term in triangular number series

1 3 6 10 15 ......... This is tirangluar number series.

The nth term of triangular series is i*(i+1)/2.

If sum S is given then to program to find the nth term is

sum = 1;
n=1;
while(sum<=S)
{
 next = ++n;
sum = sum + next;
}

So we get
value of N.