90pts求调，必关

#include<bits/stdc++.h>
#define maxn 2001000
int a[maxn];
using namespace std;
int main(){
    int n;
    long long num,ans1,ans2,b,c;
    cin>>n;
    for(int i=1;i<=n;i++){
        cin>>b>>c;
        a[b+1]++;
        a[c+1]--;
        ans1=min(ans1,b);
        ans2=max(ans2,c);
    }
    for(int i=ans1;i<=ans2;i++){
        a[i]=a[i-1]+a[i];
        if(a[i]!=0){
            num++;
        }
    }
    cout<<num;
    return 0;
}