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