#include <iostream>
#include <algorithm>
using namespace std;
struct cinema
{
    int a,b,ans;
};
int main()
{
    int n,m,ans;
    int money[10001];
    cinema C[100];
    while(cin>>n>>m)
    {
        for(int i=0;i<m;++i)
        {
           cin>>C[i].a>>C[i].b;
           C[i].ans=((n+C[i].a-1)/C[i].a)*C[i].b;
        }
        ans=1e9;
        for(int i=0;i<m;++i)
        {
            ans=min(C[i].ans,ans);
        }
        cout<<ans<<endl;
    }
    return 0;
}