usaco Mixing Milk

/*
ID: modengd1
PROG: milk
LANG: C++
*/
#include <iostream>
#include <stdio.h>
#include <queue>
struct node
{
    int P;
    int A;
    node(int p,int a)
    {
        A=a;
        P=p;
    }
    node(){}
    bool friend operator <(node n1,node n2)
    {
        return n1.P>n2.P;
    }
};
using namespace std;

int main()
{
    freopen("milk.in","r",stdin);
    freopen("milk.out","w",stdout);
    int N,M,a,b,ans;
    priority_queue<node> Q;
    scanf("%d%d",&N,&M);
    for(int i=0;i<M;i++)
    {
        scanf("%d%d",&a,&b);
        Q.push(node(a,b));
    }
    ans=0;
    while(N>0)
    {
        node now=Q.top();
        Q.pop();
        if(N>=now.A)
        {
            N-=now.A;
            ans+=now.A*now.P;
        }
        else
        {
            ans+=N*now.P;
            break;
        }
    }
    cout<<ans<<endl;
    return 0;
}