line = gets.chomp!
abort "line 1 format" unless line=~/^\d+ \d+$/
n,m = line.split.map(&:to_i)
abort "n" unless 2 <= n && n <= 10000
abort "m" unless 1 <= m && m <= n

m.times{|lno|
    line = gets.chomp!
    abort "line #{lno+2} format" unless line=~/^\d+ \d+$/
    a,b = line.split.map(&:to_i)
    abort "a [#{lno+2}]" unless 1 <= a && a <= n
    abort "b [#{lno+2}]" unless 1 <= b && b <= n
    abort "a == b [#{lno+2}]" unless a != b
}

abort "eol" unless !gets

puts "format OK"