As for the most need to calculate.
The first story begins with a 1= 1.
The opening page of the second story is a2=a 1+ 1=2.
The third one is A3 = A2+2 = A1+1+2 =1+1+2 = 4.
The fourth a4 = a3+3 =1+1+2+3 = 7.
……
The nth story begins with an = a (n-1)+(n-1) =1+1+2+...+(n-1) =1+(.
A70= 1+69*70/2 is an even number.
A69= 1+68*69/2 is an odd number 2347.