#include <iostream>
using namespace std;
int a[5], i, j, k, t;
int func_a(int x, int y, int z) {
	if (x > y) swap(x, y);
	if (x > z) swap(x, z);
	if (y > z) swap(y, z);
	if (x + y == z) return 1;
	if (x + y > z) return 2;
	if (x + y < z) return 0;
}
int main() {
	cin >> a[1] >> a[2] >> a[3] >> a[4];
	for (i = 1; i <= 4; i++) {
		for (j = 1; j <= 4; j++) {
			if (i == j) continue;
			for (k = 1; k <= 4; k++) {
				if (k == i || k == j) continue;
				t = max(t, func_a(a[i], a[j], a[k]));
			}
		}
	}
	if (t == 0) cout << "IMPOSSIBLE";
	else if (t == 1) cout << "SEGMENT";
	else if (t == 2) cout << "TRIANGLE";
	return 0;
}