Let's find out the sum of some consecutive numbers greater than 976 with the lowest difference. That is:
$1+2+3+...+44=(41*44)/2=990$
990-976=14 ; the sum of the missing page number.
Look, $14=2+12=2+3+9=2+3+4+5$ ; that we want.
So, there were 4 pages missing.
[Maybe it is the easiest way
]