#include <bits/stdc++.h>
using namespace std;
int main() {
  int a[4];
  for (int i = 0; i < 4; cin >> a[i++])
    ;
  sort(a, a + 4);
  int b = max(a[0] + a[1] - a[2], a[1] + a[2] - a[3]);
  if (b > 0)
    cout << "TRIANGLE" << endl;
  else if (!b)
    cout << "SEGMENT" << endl;
  else
    cout << "IMPOSSIBLE" << endl;
  return 0;
}
