https://codeforces.com/contest/1140/problem/C

题解：

贪心

根据b进行排序，枚举b

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=300000+10;
const int M=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k;
ll ans,cnt,flag,temp,sum;
int a[N];
char str;
struct node{
    int v,w;
    bool operator <(const node &S)const{
        return S.w== w ? v > S.v:w > S.w;
    }
}e[N];
priority_queue<int, vector<int>, greater<int> > q;
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    //ios::sync_with_stdio(false);
    //cin.tie(0);
    //cout.tie(0);
    //scanf("%d",&t);
    //while(t--){
    scanf("%d%d",&n,&k);
    for(int i=1;i<=n;i++){
        scanf("%d%d",&e[i].v,&e[i].w);
    }
    sort(e+1,e+n+1);
    for(int i=1;i<=n;i++){
		if (i <= k) {
			q.push(e[i].v);
			sum += e[i].v;
			ans = max(ans, sum*e[i].w);
		}
		else {
			int t = q.top();
			if (e[i].v > t) {
				sum -= t;
                sum += e[i].v;
				q.pop();
				q.push(e[i].v);
				ans = max(ans,e[i].w*sum);
			}
		}
	}
	printf("%lld\n", ans);

    //}

#ifdef DEBUG
	printf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC);
#endif
    //cout << "Hello world!" << endl;
    return 0;
}