19. In a "TriboNcci" sequence, each number after the third number is the sum of the preceding three numbers. For example, if the first three numbers are 5,6, and 7 , then the fourth number is 18 because 5 + 6 + 7 = 18, and the fifth number is 31 because 6 + 7 + 18 = 31. The first five numbers of another TriboNcci sequence are P, Q, 86, 158, and 291 in that order. What is the value of P ?
