Tribonacci

import java.math.BigInteger;
import java.util.Scanner;

public class Tribonacci {
    public static void main(String[] args) {
        BigInteger tN, t1, t2, t3;
        long element = 0;

        Scanner scanner = new Scanner(System.in);

        t1 = scanner.nextBigInteger();
        t2 = scanner.nextBigInteger();
        t3 = scanner.nextBigInteger();
        element = scanner.nextLong();

        tN = Tribonacci(t1, t2, t3, element);
        System.out.println(tN);
    }

    public static BigInteger Tribonacci(BigInteger t1, BigInteger t2, BigInteger t3, long element) {
        BigInteger tN = null;

        if ( element < 4){
            if ( element ==  1) { return t1;
            }else if ( element == 2) { return t2;
            }else if ( element == 3) { return t3;}
            }else {
                for (int i = 0; i < element - 3; i++) {
                    tN = t1.add(t2).add(t3);
                    t1 = t2;
                    t2 = t3;
                    t3 = tN;
                }
            }
        return t3;
    }
}