74分求助

#include<bits/stdc++.h>
using namespace std;
int n,m;
struct cow{
	int price,num;
}c[5005];
bool cmpc(cow x,cow y)
{
	return x.price<y.price;
}
int main()
{
	scanf("%d%d",&n,&m);
	for(int i=1;i<=m;i++)
	    scanf("%d%d",&c[i].price,&c[i].num);
	sort(c+1,c+m+1,cmpc);
	long long money=0;
	for(int i=1;i<=n;i++)
	{
		if(n<c[i].num)
		{
			money+=c[i].price*(c[i].num-n);
			break;
		}
		money+=c[i].price*c[i].num;
		n-=c[i].num;
	}
	printf("%lld",money);
	return 0;
}