Set a Boolean variable "first" to true.
While input value is not negative
If first is true
If input value is positive
Set the minimum to the input value.
Set first to false.
Else
Output 0 is not a valid input.
Else
If input value is positive
If the input value is less than the minimum
Set the minimum to the input value.
Else
Output 0 is not a valid input.
If first is true
Output no valid numbers were input so there is not a minimum value.
Else
Output the minimum value.