简单的贪心,将cent从小到大排序.
/*
ID:wang9621
PROG:milk
LANG:C++
*/
#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
struct edge
{
int cent;
int unit;
};
edge per[];
bool cmp(edge a,edge b)
{
return a.cent<b.cent;
}
int main()
{
freopen("milk.in","r",stdin);
freopen("milk.out","w",stdout);
int n,m;
scanf("%d %d",&n,&m);
for(int i = ; i<=m; i++)
{
scanf("%d %d",&per[i].cent,&per[i].unit);
}
sort(per+,per+m+,cmp);
int sum = ;
for(int i = ; i<=m; i++)
{
if(n>=per[i].unit)
{
sum += per[i].cent*per[i].unit;
n -= per[i].unit;
}
else if(n>)
{
sum += n*per[i].cent;
n = ;
} }
printf("%d\n",sum);
return ;
}