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