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