Unequal Coins   (coins.X)

Bessie has discovered treasure buried at the farm! The
long-buried chest contains N coins (1 <= N <= 1,000) numbered 1..N.
Farmer Ran, wanting to determine how many distinct coin types
are in the find, has conducted W (1 <= W <= 3,000) weighings of the
coins to deduce ordered pairs (A,B) which denote that coin A is
heavier than coin B.

Bessie wants to find the "most distinct" coin -- the one that is 
_necessarily_ different from the largest
number of other coins. If there are multiple solutions, print the coin
with the smallest number. If the given input contains impossible 
contradictions, print "IMPOSSIBLE".

PROBLEM NAME: coins.X

INPUT FORMAT:

* Line 1: Two space-separated integers: N and W.

* Lines 2..W+1: Each line contains two space-separated integers A and
        B and denotes that coin A weighs more than coin B.

SAMPLE INPUT:

7 6
1 6
1 5
3 6
4 3
2 4
2 5

OUTPUT FORMAT:

* Line 1: The smallest coin number of a coin that differs from the
        largest number of other coins.

SAMPLE OUTPUT:

2

OUTPUT DETAILS:

We have 7 coins and 6 inequalities. 2 is not equal to 3, 4, 5 and 6.