Subtracting cells but Excel adds them up

Say investing activities at G3 are -2300

But the return from financing activities at G4 is 3000

So increase/decrease from financing activities shown in G5 should be 700

My problem is that Excel adds both G3 and G4 and gives a total in G5 as 5300

I have tried =G4-G3 and =SUM(G4-G3) 

But same result 5300

And if I try =G3-G4 and =SUM(G3-G4)  I get the result -5300

Excel seems to think that if the first cell in the equation is a negative the second must be as well, and vice versa

Thank You