大佬们，蒟蒻30分WA求助

#include<bits/stdc++.h>
using namespace std;
struct building{
	int need,last;
}arr[200000];
bool cmp(building a,building b){
	if(a.last==b.last) return a.need>b.need;
	else return a.last<b.last;
}
int n,num;
int time_,k=1;
int main(){
	cin>>n;
	for(int i=1;i<=n;i++) cin>>arr[i].need>>arr[i].last;
	sort(arr+1,arr+1+n,cmp);
	while(n--){
		if(time_+arr[k].need<=arr[k].last){
			num++;
			time_+=arr[k].need;
		}
		k++;
	}
	cout<<num;
	return 0;
}

求求各位大佬们帮帮此蒟蒻调试一下（qwq）