W72 求助

#include<bits/stdc++.h>
using namespace std;
int n,l[20005],r[20005];
long long sum=0;
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>l[i]>>r[i];
	}
	int j=1;
	int f=0;
	for(int i=1;i<=n;i++){
		if(j<=l[i]){
			sum+=(r[i]-j);
			sum++;
			j=r[i];
		}else if(j>=r[i]){
			sum+=(j-l[i]);
			sum++;
			j=l[i];
		}else{
			if(r[i]-j<j-l[i]){
				sum+=(r[i]-j);
				sum+=(r[i]-l[i]);
				sum++;
				j=l[i];
			}else{
				sum+=(j-l[i]);
				sum+=(r[i]-l[i]);
				sum++;
				j=r[i];
			}
		}
	}
	cout<<sum+n-j-1;
	return 0;
}