For the below code, I keep getting the result 0d0. I am expecting to get 1.3515415d0. Not sure where the bug is.

```
program test
print *, call_price(n = 5, s = 10d0, sigma = 2d-1, t = 1d0, r = 1d-1, k = 10d0)
contains
function call_price(n, s, sigma, t, r, k)
integer(4) :: n, i1, i2
real(8) :: s, sigma, t, r, k, call_price, delta_t, u, d, q, stock_tree(n + 1, n + 1), option_tree(n + 1, n + 1)
delta_t = t/n
stock_tree = 0d0
u = exp(sigma*sqrt(delta_t))
d = exp(-sigma*sqrt(delta_t))
do i1 = 1, n + 1
do i2 = 1, i1
stock_tree(i1, i2) = s*u**(i2 - 1)*d**((i1 - 1) - (i2 - 1))
end do
end do
q = (exp(r*delta_t) - d)/(u - d)
option_tree = 0d0
do i2 = 1, n + 1
option_tree(n + 1, i2) = max(stock_tree(n + 1, i2) - k, 0d0)
end do
do i1 = n, 1
do i2 = 1, i1
option_tree(i1, i2) = ((1d0 - q)*option_tree(i1 + 1, i2) + q*option_tree(i1 + 1, i2 + 1))/exp(r*delta_t)
end do
end do
call_price = option_tree(1, 1)
end function
end program
```