#include<bits/stdc++.h>
using namespace std;
struct mode{
	int zg,mike;
}a[2001000];
bool cmp(mode x,mode y)
{
	return x.zg<y.zg;
}
int k,n,sum;
int main(){
	cin>>k>>n;
	for(int i=1;i<=n;i++) 
	{
		cin>>a[i].zg>>a[i].mike;
	}
	sort(a+1,a+1+n,cmp);
	for(int i=1;i<=n;i++)
	{
		if(k>=a[i].mike)
		{
			k-=a[i].mike;
			sum+=a[i].zg*a[i].mike;
			continue;
		}
		if(k==0) break;
		else sum+=k*a[i].zg,k=0;
	}
	cout<<sum;
	return 0;
}

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ok,代码

#include<bits/stdc++.h>
using namespace std;
int n,m,s;
struct Node
{
    int p,a;
}z[6000];
bool cmp(Node x,Node y)
{
    return x.p<y.p;
}
int main()
{
    cin>>n>>m;
    for(int i=1;i<=m;i++)
    {
        cin>>z[i].p>>z[i].a;
    }
    sort(z+1,z+m+1,cmp);
    for(int i=1;i<=m;i++)
    {
        if(z[i].a<n)
        {
            s+=z[i].p*z[i].a;
            n-=z[i].a;
        }
        else
        {
            s+=z[i].p*n;
            break;
        }
    }
    cout<<s<<endl;
    return 0;
}

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