Assume that objects a1, a2, and a3 are managed by a server, which provides two operations to operate the objects. read(a): returns the value of object a write(a, v): assigns the value v to object a Assume that the following two concurrent transactions T and U are performed on these objects. T: read(a2); read(a1); write(a2, a2-25); read(a3); write(a1, a1+52) U: read(a3); read(a2); write(a2, a2+33); write(a3, a3-26) Assume that the original values of a1, a2, and a3 are 111, 106 and 125 respectively.

T: read(a2); U: read(a3); U: read(a2); T: read(a1); T: write(a2, a2-25); T: read (a3); U: write(a2, a2+33); T: write(a1, a1+52); U: write(a3, a3-26)

Question:

What requirement must be satisfied in order to avoid the problem?