最后一个点WA，求条

#include<bits/stdc++.h>
using namespace std;
struct saler{
	int price;
	int number;
};
bool cmp(saler a,saler b){
	return a.price<b.price;
}
int main(){
	int m,n,nowget=0,sum=0;
	cin>>n>>m;
	saler a[m];
	for(int i=0;i<m;i++){
		cin>>a[i].price>>a[i].number;
	}
	sort(a,a+m,cmp);
	for(int i=0;i<n;i++){
		if(nowget==n) break;
		if(a[i].number<=(n-nowget)){
			nowget+=a[i].number;
			sum+=a[i].number*a[i].price;
		}
		if(a[i].number>(n-nowget)){
			sum+=(n-nowget)*a[i].price;
			break;
		}
	}
	cout<<sum;
	return 0;
}