Decrement a dataframe column based on a condition

Question

If I have a dataframe like this how can I decrement 'a' based on 'b'?

df = pd.DataFrame([[100,1], [100,2], [100,3], [100,4], [100,5]], columns=["a", "b"])
df.loc[df.b > 2, 'b'] += 1
df.loc[df.b % 2 == 0, 'a'] -= 10
df.loc[df.b % 2 != 0, 'a'] = np.NaN

Desired result:

   a    b
0  NaN  1
1   90  2
2   80  4
3  NaN  5
4   70  6