Time Limit: 1000MS Memory Limit: 65536K Total Submissions: 4491 Accepted: 2325

###### Description

There is a sequence of n+2 elements a0, a1, …, an+1 (n <= 3000, -1000 <= ai <=1000). It is known that ai = (ai-1 + ai+1)/2 - ci for each i=1, 2, …, n.
You are given a0, an+1, c1, … , cn. Write a program which calculates a1.

###### Input

The first line of an input contains an integer n. The next two lines consist of numbers a0 and an+1 each having two digits after decimal point, and the next n lines contain numbers ci (also with two digits after decimal point), one number per line.

###### Output

The output file should contain a1 in the same format as a0 and an+1.

150.5025.5010.15

###### Sample Output

27.85

ai+1 - ai = a1- a0 + 2(c1+c2+c3+…………..+ci) ai - ai-1 = a1- a0 + 2(c1+c2+c3+……….+ci-1) ai-1 - ai-2 = a1- a0 + 2(c1+c2+c3+….+ci-2) . . . . . . . . . . . . . . . . .. . . . . a2 - a1 = a1- a0 + 2c1相加化简得： ai+1 - a1= a1- a0 +2( nc1 + (n-1)c2 + (n-2)c3 +…..+2cn-1+cn )

c2 + (n-2)c3 +…..+2cn-1+cn ))/(n+1)