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