#include <bits/stdc++.h>
using namespace std;
int main() {
  int a[5];
  for (int i = 0; i < 4; i++) cin >> a[i];
  sort(a, a + 4);
  if ((a[3] < (a[2] + a[1])) || (a[2] < (a[1] + a[0])))
    cout << "TRIANGLE" << endl;
  else if ((a[3] == (a[2] + a[1])) || (a[2] == (a[1] + a[0])))
    cout << "SEGMENT" << endl;
  else
    cout << "IMPOSSIBLE" << endl;
  return 0;
}
