简单贪心 没什么好说的
/*
ID: Jang Lawrence
PROG: milk
LANG: C++
*/
#include<iostream>
#include<cstring>
#include<cstdio>
#include<algorithm>
using namespace std;
int n,m;
struct
ps
{
int p,num;
}a[5010];
bool cmp(ps p,ps q)
{
return p.p<q.p;
}
int main()
{
freopen("milk.in","r",stdin);
freopen("milk.out","w",stdout);
scanf("%d%d",&n,&m);
for(int i=1;i<=m;++i)
scanf("%d%d",&a[i].p,&a[i].num);
sort(a+1,a+1+m,cmp);
long long ans=0;
int i=1;
while(n)
{
if(n>=a[i].num)
{
n-=a[i].num;
ans+=a[i].p*a[i].num;
}
else
{
ans+=n*a[i].p;
n=0;
}
++i;
}
printf("%lld\n",ans);
return 0;
}