70分求助，玄关

#include<bits/stdc++.h>
using namespace std;
int n;
int a[20005],b[20005];
int f[100000005];
long long l;
long long num;
int main(){
	cin >> n;
	for(int i = 1;i <= n;i++){
		cin >> a[i] >> b[i]; 
		for(int j = a[i]+1;j <= b[i];j++){
			f[++l] = j;
		}
	}
	sort(f+1,f+l+1);
	int s = 0;
	for(int i = 1;i <= l;i++){
		if(i == 1){
			num++;
			s = f[i];
		} 
		if(f[i] != s){
			s = f[i];
			num++;
		}
	}
	cout << num;
	return 0;
}