#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[0] + a[1] > a[2] || a[1] + a[2] > a[3]) {
    cout << "TRIANGLE" << endl;
    return 0;
  }
  if (a[0] + a[1] == a[2] || a[1] + a[2] == a[3]) {
    cout << "SEGMENT" << endl;
    return 0;
  } else {
    cout << "IMPOSSIBLE" << endl;
  }
}
