hoj2682Magic-Pen2

//代码含全角空格
#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;
const int N=10005;
int a[N],b[N],n,m,x,y;

int main(){
    while(scanf("%d%d",&n,&m)==2){
        memset(a,0,sizeof(a));
        memset(b,0,sizeof(b));
        while(m--){
            scanf("%d%d",&x,&y);
            b[x]++;b[y+1]--;
        }
        int sum=0;
        for(int i=1;i<=n;i++){
            a[i]=a[i-1]+b[i];
            if(a[i]&1) a[i]=1;      
            else {
                sum++;
                a[i]=0;
            }
        }
        printf("%d\n",sum);
    }
    return 0;
}