71. In a junior high school graduation class, 100 students and teachers together are taking a graduation photo on the steps. The photographer needs to arrange them in a trapezoidal formation with more people in the front rows than in the back rows (number of rows $\geqslant 3$), and requires that the number of people in each row must be consecutive natural numbers, so that the people in the back row can stand in the gaps between the people in the front row. How many different formations meet these conditions? $\qquad$
answer: 2