text: # coding: utf-8
  from itertools import combinations
  l = [int(i) for i in input().split()]
  comb = list(combinations([0,1,2,3],3))
  for c in comb:
      if l[c[0]]+l[c[1]]>l[c[2]] and\
         l[c[1]]+l[c[2]]>l[c[0]] and\
         l[c[0]]+l[c[2]]>l[c[1]]:
          print('TRIANGLE')
          exit()
  for c in comb:
      if l[c[0]]+l[c[1]]==l[c[2]] or\
         l[c[1]]+l[c[2]]==l[c[0]] or\
         l[c[0]]+l[c[2]]==l[c[1]]:
          print('SEGMENT')
          exit()
  print('IMPOSSIBLE')
  