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