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