#include <bits/stdc++.h>

using namespace std;

int main()
{
	int a[4], i;

	for (i = 0; i < 4; i++)
		scanf("%d", &a[i]);

	sort(a[0], a[3]);
	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;

	return 0;
}