To have a minus altitude or negative altitude you need to go below 0 ( which is represented by the sea level).
To get the difference between the mountain climber and submarine commander you add both altitudes as positive numbers: 4572 + 609 = 5181 m difference!
Another ex.
If a scuba diver is under the water, say 10 m hhe has a altitude of -10m
And a submarine is under at 100m that is a altitude of -100m.
The difference between the two is 100-10=90 m, not -90m. The difference is always a positive number.
