Given the following declaration statements:

// Assume the 64-bit GCC compiler provides storage for array object a at address 1000
short a[] = { 3, 6, 2, 4, 7, 8 };
short *p1 = a + 1, *p3 = a + 3, *p5 = a + 5;

evaluate expression p1 += p5 - p3. If the expression cannot be compiled, write [for compile-time error]. If the expression generates undefined behavior, write [for undefined behavior]. Otherwise, write the exact value resulting from the expression's evaluation.

Brief side-note on undefined behavior: The C standard says that statements such as c = (b = a + 2) - (a = 1); and c = (b = a + 2) - (a = 1); cause undefined behavior [because we don't know whether the left or right operand of operator - is evaluated first]. When a program ventures into the realm of undefined behavior, all bets are off. The program may behave differently when compiled with different compilers. But that's not the only thing that can happen. The program may not compile in the first place, if it compiles it may not run, and if it does run, it may crash, behave erratically, or produce meaningless results. In other words, undefined behavior should be avoided like the plague.