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