CodeForces-1132C-Painting the Fence-（前缀和）

You have a long fence which consists of nn sections. Unfortunately, it is not painted, so you decided to hire qq painters to paint it. ii-th painter will paint all sections xx such that li≤x≤rili≤x≤ri.

Unfortunately, you are on a tight budget, so you may hire only q−2q−2 painters. Obviously, only painters you hire will do their work.

You want to maximize the number of painted sections if you choose q−2q−2 painters optimally. A section is considered painted if at least one painter paints it.

Input

The first line contains two integers nn and qq (3≤n,q≤50003≤n,q≤5000) — the number of sections and the number of painters availible for hire, respectively.

Then qq lines follow, each describing one of the painters: ii-th line contains two integers lili and riri (1≤li≤ri≤n1≤li≤ri≤n).

Output

Print one integer — maximum number of painted sections if you hire q−2q−2painters.

Examples

Input

7 5
1 4
4 5
5 6
6 7
3 5

Output

7

Input

4 3
1 1
2 2
3 4

Output

2

Input

4 4
1 1
2 2
2 3
3 4

Output

3
解题过程：
遍历一下每个栅栏有多少人会在这里刷漆，对于在漆工范围内的栅栏存储总量。用前缀和数组记录刷一次和刷两次的位置。
暴力寻找哪两个漆工去掉后会让栅栏不能刷漆，取最小值。用总量-最小值。大牛只需要分三种情况，在下天资愚钝，分了6种情况。
第一种：
.........................
.........................
第二种：
.......................
..............................
第三种：
.........................
    ..............
第四种：
.........................
       ..................
第五种：
.........................
     ...........................
第六种：
.............
                  ............
上代码：

#include<stdio.h>
#include<math.h>
#include<string.h>
#include<algorithm>
#include<string>
#include<vector>
#include<iostream>
#include<cstring>
#define inf 0x3f3f3f3f
using namespace std;
#define ll long long

struct node
{
    int l;
    int r;
    int idx;
};
int x,y;
node a[5005];
int t[5005];
int one[5005];
int two[5005];///前缀和数组

bool cmp(node p1,node p2)
{
    if(p1.l==p2.l)
        return p1.r<p2.r;
    else return p1.l<p2.l;
}

int main()
{
    int n,m;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        memset(a,0,sizeof(a));
        memset(t,0,sizeof(t));
        for(int i=0;i<m;i++)
        {
            scanf("%d%d",&a[i].l,&a[i].r);
            for(int j=a[i].l;j<=a[i].r;j++)
                t[j]++;
        }
        int ans=0,minn=inf;
        for(int i=1;i<=n;i++)
        {
            if(t[i])
            {
                ans++;
            }
            one[i]=one[i-1]+(t[i]==1);///前缀和赋值不能放括号里面
            two[i]=two[i-1]+(t[i]==2);///前缀和需要连续赋值，如果if不成功则不赋值，造成中断
        }
        sort(a,a+m,cmp);
        for(int i=0;i<m;i++)///hello
        {
            for(int j=i+1;j<m;j++)///下标j在i后面，j的l始终大于等于i的l
            {
                if( a[i].l==a[j].l && a[i].r==a[j].r)///1
                    minn=min(minn,( two[ a[i].r ]-two[ a[i].l-1 ] ) );
                else if(a[i].l==a[j].l && a[i].r<a[j].r)///2
                    minn=min(minn,( two[ a[i].r ]-two[ a[i].l-1 ] ) + ( one[ a[j].r ]-one[ a[i].r ] ) );
                else if( a[i].r>a[j].r )///3
                    minn=min(minn,( one[ a[j].l-1 ]-one[ a[i].l-1 ] ) + ( two[ a[j].r ]-two[ a[j].l-1 ] ) + ( one[ a[i].r ]-one[ a[j].r ])  );
                else if( a[i].r==a[j].r )///4
                    minn=min(minn,( one[ a[j].l-1 ]-one[ a[i].l-1 ] ) + ( two[ a[j].r ]-two[ a[j].l-1 ] )  );
                else if( a[i].r<a[j].r && a[i].r>=a[j].l )///5
                    minn=min(minn,( one[ a[j].l-1 ]-one[ a[i].l-1 ] ) + ( two[ a[i].r ]-two[ a[j].l-1 ] ) + ( one[ a[j].r ]-one[ a[i].r ])  );
                else if( a[i].r<a[j].l )///6
                    minn=min(minn,( one[ a[i].r ]-one[ a[i].l-1 ] ) + ( one[ a[j].r ]-one[ a[j].l-1 ] )  );
            }
        }
        printf("%d\n",ans-minn);

    }
    return 0;
}