#include <fstream>
    #include <algorithm>
    using namespace std;
    class abc
    {
    public:
    int jg,jl;
    };
    int main()
    {
    ifstream in("milk.in");
    ofstream out("milk.out");
    int b,c,d,e=0,t=1,g,n,m,x,y=0,i,f,j;
    abc a[9998];
    in>>n>>m;
    for(i=1;i<=2*m;i++)
    {
        in>>a[i].jg>>a[i].jl;
    }
     
    for(i=1;i<=m-1;i++)
    {
        x=a[i].jg;b=i;
       for(j=i;j<=m;j++)
    {
         if(a[j].jg<x)
         {
            x=a[j].jg;
            b=j;
          }
          }
            c=a[i].jg;a[i].jg=a[b].jg;a[b].jg=c;g=a[i].jl; a[i].jl=a[b].jl;a[b].jl=g;
    }
    for(i=1;i<=4;i++)
    {
           y=a[i].jg*a[i].jl+y;
          e=a[i].jl+e;
        if(e==n)
          break;
       if(e>n)
    {
         f=e-n;
         y=y-a[i].jg*f;
       break;
    }
    }
    out<<y;
    in.close();
    out.close();
    return 0;
    }