#include <bits/stdc++.h>
using namespace std;
long a[10];
int main() {
  for (long k = 0; k < 4; k++) cin >> a[k];
  sort(a, a + 4);
  if (a[0] + a[1] > a[2] || a[1] + a[2] > a[3]) {
    cout << "TRIANGLE" << endl;
  } else if (a[0] + a[1] == a[2] || a[1] + a[2] == a[3]) {
    cout << "SEGMENT" << endl;
  } else
    cout << "IMPOSSIBLE" << endl;
}
