76分RE#2#3两点求助

#include <bits/stdc++.h>
#define ll long long
using namespace std;
const int N=5005;
struct node
{
	int p,a;
} b[N];
bool cmp(node X,node Y)
{
	return X.p<Y.p;
}
ll n,m,ans;
int main()
{
	ios::sync_with_stdio(0);
	cin>>n>>m;
	for(int i=1;i<=m;i++) cin>>b[i].p>>b[i].a;
	sort(b+1,b+1+m,cmp);
	int i=1;
	while(n-b[i].a>=0)
	{
		ans+=b[i].p*b[i].a;
		n-=b[i++].a;
	}
	ans+=b[i].p*n;
	cout<<ans;
	return 0;
}