package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
)

func main() {
	in := bufio.NewReader(os.Stdin)
	a := make([]int, 4)
	for i := 0; i < 4; i++ {
		fmt.Fscan(in, &a[i])
	}

	hasSegment := false

	for i := 0; i < 4; i++ {
		for j := i + 1; j < 4; j++ {
			for k := j + 1; k < 4; k++ {
				b := []int{a[i], a[j], a[k]}
				sort.Ints(b)
				if b[0]+b[1] > b[2] {
					fmt.Println("TRIANGLE")
					return
				}
				if b[0]+b[1] == b[2] {
					hasSegment = true
				}
			}
		}
	}

	if hasSegment {
		fmt.Println("SEGMENT")
	} else {
		fmt.Println("IMPOSSIBLE")
	}
}