Is this a sign of my excel program being corrupt or something else?

Whenever i change a value that is involved in a formula, the result does not
update with the new value.

For example, a very basic example is if i put 50 in A1 and 50 in A2, then
create a formula in another cell to be A1+A2 the result in that cell would be
100. Now if i change the value in A1 to be 25, the formula cell should
change to read 75. Mine does not automatically - it still reads 100 after
the change even thought clearly A1 + A2 should read 75. I have to retype in
my formula a second time in order for the value update to occur.

Please if you someone could offer some feedback. Thanks.

Andrew K.