求助大佬

#include<bits/stdc++.h>
 using namespace std;
 struct opp
 {
 	int id,ans,dan;
 }a[5001];
bool cmp(opp x,opp y)
{
 return x.dan<y.dan;
}
int n,m;
int main()
{
	scanf("%d%d",&n,&m);
	for(int i=1;i<=n;i++)
	{
		scanf("%d",&a[i].dan);
		scanf("%d",&a[i].ans);
		a[i].id=i;
	}
	short(a+1,a+n,cmp)
	int ans1=0,ans2=0,i=1;
	while(ans1<=n)
	{
		ans1+=a[i].ans;
		ans2+=a[i].dan*a[i].ans;
		i++;
	}
	cout<<ans2;
	return 0;
}