贪心，刚做此题时，就意识到了是贪心，但贪心思路一直没找对,wa了n次，在纠结中终于想出正确的思路
#include <iostream>
#include <algorithm>
#include <cstdio>
using namespace std;
struct node
{
int x,y;
};
node f[25];
bool cmp(node a,node b)
{
return a.y*b.x<b.y*a.x;
}
int main()
{
int n;
while(cin>>n)
{
int i;
long long amount=0,tot=0;
for(i=0;i<n;i++)
{
scanf("%d%d",&f[i].x,&f[i].y);
amount+=f[i].x;
}
sort(f,f+n,cmp);
for(i=0;i<n;i++)
{
tot+=amount*f[i].y;
amount-=f[i].x;
}
cout<<tot<<endl;
}
return 0;
}