P1208 [USACO1.3] 混合牛奶 Mixing Milk

#1、#7、#8 WA掉了，$62$分

#include <iostream>
#include <algorithm>
using namespace std;
struct M
{
	int p,a;
}b[5010];
bool cmp(M x,M y)
{
	return x.p<y.p;
}
int main() 
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n,m,sum=0;
	cin>>n>>m;
	for (int i=1;i<=m;i++)
	{
		cin>>b[i].p>>b[i].a;
	}
	sort(b+1,b+m+1,cmp);
	for (int i=1;i<=m;i++)
	{
		if (n-b[i].a>=0)
		{
			sum+=b[i].a*b[i].p;
			n-=b[i].a;
		}
		else
		{
			sum+=n*b[i].p;
		}
	}
	cout<<sum<<'\n';
	return 0;
}