rt,本人AC代码在输入样例4时输出为889，代码如下

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int MAXN=1000001;

int n;
int a[MAXN],cha[MAXN],dp[MAXN];
signed main(){
	freopen("increase.in","r",stdin);
	freopen("increase.out","w",stdout);
	ios::sync_with_stdio(0);
	cin.tie(0),cout.tie(0);
	cin>>n;
	for (int i=1;i<=n;i++){
		cin>>a[i];
		cha[i] = a[i]-a[i-1];
	}
	cha[n+1]=1;//注释此行代码则样例全过但WA，保留此行代码则AC但样例四无法通过
	for (int i=1;i<=n;i++){
		if (cha[i]==1&&cha[i+1]==1){
			dp[i] = dp[i-1]+1;
		}else dp[i] = dp[i-1];
	}
	cout<<dp[n];
	return 0;
}

该代码在输入： 2 1 2 输出： 2 可见测试点过弱