Codeforces Round #FF (Div. 2):C. DZY Loves Sequences

C. DZY Loves Sequences
time limit per test
1 second
memory limit per test
256 megabytes
input
standard input
output
standard output
DZY has a sequence a, consisting of n integers.
We'll call a sequence ai, ai + 1, ..., aj (1 ≤ i ≤ j ≤ n) a subsegment of the sequence a. The value (j - i + 1) denotes the length of the subsegment.
Your task is to find the longest subsegment of a, such that it is possible to change at most one number (change one number to any integer you want) from the subsegment to make the subsegment strictly increasing.
You only need to output the length of the subsegment you find.
Input
The first line contains integer n (1 ≤ n ≤ 105). The next line contains n integers a1, a2, ..., an (1 ≤ ai ≤ 109).
Output
In a single line print the answer to the problem — the maximum length of the required subsegment.
Sample test(s)
Input

6
7 2 3 1 5 6

Output

5

Note
You can choose subsegment a2, a3, a4, a5, a6 and change its 3rd element (that is a4) to 4.
제목은 문자열의 한 문자를 바꾸어 가장 긴 연속 문자열을 낼 수 있다는 것이다
안건대로 7 2 3 1 5 6 ---7 2 3 4 5 6
출력은 5입니다.

#include
#include
#include
#include
#include
#include
#include
#include

using namespace std;

#define f1(i, n) for(int i=0; i=1; i--)
#define f4(i, n) for(int i=1; i<=n; i++)
#define f5(i, n) for(int i=2; i<=n; i++)
#define M 1005

const int INF = 0x3f3f3f3f;
int n, a[100005], b[100005];

int main()
{
    cin>>n;
    f4(i, n)
    cin>>a[i];
    b[1]=1;
    f5(i, n)
    {
        b[i]=1;
        if (a[i]>a[i-1])
            b[i]=b[i-1]+1;
    }
    int ans=-INF;
    f3(i, n)
    {
        if (b[i]==n)
            ans=max(b[i], ans);
        else
            ans=max(ans, b[i]+1);
        if (a[i-b[i]+1]-1>a[i-b[i]-1])
            ans=max(ans,b[i]+b[i-b[i]]);
        if (a[i-b[i]+2]-1>a[i-b[i]])
            ans=max(ans,b[i]+b[i-b[i]]);
    }
    cout<