Strange arithmetic problem

Hello, Recently, I am developing some application. When it came to implement OnMouseWheel function I came accros this problem. Here is the code:

// get scrollbar info
si.cbSize = sizeof( SCROLLINFO );
si.fMask  = SIF_ALL;
GetScrollInfo( SB_VERT, &si );

// get change ration
si.fMask = SIF_POS;

// THIS IS THE LINE WHICH PRODUCES THE WRONG RESULT!!!
si.nPos  -= ( zDelta/WHEEL_DELTA )*si.nPage/2;

SetScrollInfo( SB_VERT, &si, TRUE );

When: zDelta = -120 WHEEL_DELTA = 120 si.nPage = 263 I got result = -2147483516 but when I change that line into:

si.nPos  -= ( zDelta/WHEEL_DELTA )*( si.nPage/2 );

everything is ok. Why is it so ? I guess there is sth to do with arithmetic operator priorites, Am I right ? Thx