Mixing Milk

/*
    PROB:milk
    ID:wanghan
    LANG:C++
*/
#include "iostream"
#include "cstdio"
#include "cstring"
#include "string"
#include "algorithm"
using namespace std;
const int maxn= 5000+10;
struct Node{
    int p,a;
};
Node h[maxn];
bool cmp(Node x,Node y){
    if(x.p==y.p)
        return x.a>y.a;
    return x.p<y.p;
}
int m;
long long n;
int main()
{
    freopen("milk.in", "r", stdin);  
    freopen("milk.out", "w", stdout);
    cin>>n>>m;
    for(int i=0;i<m;i++)
        cin>>h[i].p>>h[i].a;
    sort(h,h+m,cmp);
    long long ans=0;
    int flag=0;
    /*if(n>=h[0].a){
        n-=h[0].a;
        ans+=(h[0].p*h[0].a);
        if(n==0) flag=1;
    }else{
        flag=1;
        ans+=(h[0].a-n)*h[0].p;
        //n=-1;
    }*/
    //cout<<ans<<endl;
    for(int i=0;i<m;i++){
        if(flag)  break;
        if(n>=h[i].a){
            n-=h[i].a;
            ans+=(h[i].a*h[i].p);
            if(n==0)  flag=1;
        }else{
            ans+=n*h[i].p;
            n=-1;
            flag=1;
        }
    }
    cout<<ans<<endl;
}