#include <cstdio>
#include <iostream>
#include <string>
#include <sstream>
#include <cstring>
#include <stack>
#include <queue>
#include <algorithm>
#include <cmath>
#include <map>
#define PI acos(-1.0)
#define ms(a) memset(a,0,sizeof(a))
#define msp memset(mp,0,sizeof(mp))
#define msv memset(vis,0,sizeof(vis))
using namespace std;
//#define LOCAL
struct Node
{
    int num;//绿豆数量
    int need;//需要猫粮
}t;
int cmp(Node n1,Node n2)//单价从小到大
{
    return 1.0*n1.need/n1.num<1.0*n2.need/n2.num;
}
int main()
{
#ifdef LOCAL
    freopen("in.txt", "r", stdin);
#endif // LOCAL
    ios::sync_with_stdio(false);
    int n,m;//猫粮,组数
    vector<Node> v;
    while(cin>>n>>m&&n!=-1)
    {
        v.clear();
        double ans=0;
        for(int i=0;i<m;i++)
        cin>>t.num>>t.need,v.push_back(t);
        sort(v.begin(),v.end(),cmp);
        for(int i=0,siz=v.size();i<siz;i++)
        {
            t=v[i];
            if(t.need<=n)ans+=t.num,n-=t.need;
            else {ans+=n/(1.0*t.need/t.num);break;}
        }
        printf("%.3lf\n",ans);
    }
    return 0;
}