HRBUST 1909 理工门外的树

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
int fi[1000010];
int main()
{
    int n,m;
    while(~scanf("%d%d",&n,&m))
    {
        memset(fi,0,sizeof(fi));
        int sum = 0;
        int ans = 0;
        while(m--)
        {
            int l,r;
            scanf("%d%d",&l,&r);
            fi[l]++,fi[r+1]--;
        }
        for(int i = 0; i <= n; i++)
        {
            sum += fi[i];
            if(sum == 0)
                ans++;
        }
        printf("%d\n",ans);
    }
    return 0;
}