求指出哪有问题（请勿发新代码，求帮忙知错改正）

#include<bits/stdc++.h>
using namespace std;

struct cow{
	int price,amount;
};
cow a[5005];

bool cmp(cow a,cow b){
	return a.price<b.price;
}

int main(){
	int n,m,money=0;
	cin>>n>>m;
	for(int i=1;i<=m;i++)cin>>a[i].price>>a[i].amount;
	sort(a+1,a+m+1,cmp);
	for(int i=1;;i++){
		if(n-a[i].amount>0){
			n-=a[i].amount;
			money+=a[i].amount*a[i].price;
		}
		else {
			money+=(a[i].amount-n)*a[i].price;
			break;
		}
	}
	cout<<money;
	return 0;
}