#include <iostream>

using namespace std;

bool ist( int a, int b, int c )
{
    int aux = max(a,b);
    aux = max(aux,c);
    int s = a + b + c;
    if ( s - aux >= aux )
        return true;
    return false;
}

int main()
{
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int r = 0;
    if ( ist(a,b,c) )
        r++;
    if ( ist(a,b,d) )
        r++;
    if ( ist(a,c,d) )
        r++;
    if ( ist(b,c,d) )
        r++;
    if ( r == 1 )
        cout << "SEGMENT";
    else if ( r == 0 )
        cout << "IMPOSSIBLE";
    else
        cout << "TRIANGLE";
    return 0;
}