```
#include<bits/stdc++.h>
using namespace std;
int n,m,v[5004],num[100070],c,sum = 0;
int main()
{
cin>>n>>m;
for(int i = 1;i <= m;i++)
{
cin>>v[i];//单价和对应数量
cin>>c;
num[v[i]] += c;
}
sort(v+1,v+m+1);
for(int i = 1;i <= m;i++)
{
if(num[v[i]] == 0)
continue;
if(n <= num[v[i]])
{
sum += v[i]n;
break;
}
else
{
sum += v[i]*num[v[i]];
n =n - num[v[i]];
}
}
cout<<sum<<endl;
return 0;
}
```
by z2441784079 @ 2019-03-12 23:08:04