Arduino Rocket Ejection at Certain Altitude

Projects General Guidance
50

Sure I will post pictures!
This is the code im using. Im getting an error at the highlighted place, And I cant solve it no matter what i do.
#include <Wire.h>
#include <BMP085.h>

Adafruit_BMP085 bmp;

unsigned long datum;
unsigned long currPressure;

void setup()
{
Wire.begin();
bmp.begin();
datum = bmp.readPressure();
}

void loop()
{
currPressure = bmp.readPressure();
if ((datum - currPressure) > 28)
{
// Deploy 'chute...
}
}