System.out.printf("%.3f",maxF);//maxF为输出结果 即最大鼠食

System.out.println();

import java.util.*;

import java.io.*;

/**
*数组实现，实现当中运用了快排，具体见static方法
*/

public class Main{

    public static void main(String[] arg){

        Scanner scan = new Scanner(new BufferedInputStream(System.in));

        int m,n;

        while((m=scan.nextInt())!=-1&&(n=scan.nextInt())!=-1){

            double f[] = new double[n];//鼠食

            double j[] = new double[n];//猫粮

            double a[] = new double[n];

            double maxF=0;

            for(int i =0 ; i != n ; i ++){

                f[i] = scan.nextDouble();

                j[i] = scan.nextDouble();

                a[i] = f[i]/j[i];

            }

            sort(a,f,j);

            for(int i =n-1 ; i !=-1 ; i -- ){

                if(m>=j[i]){//这里必须打等于号  不然会导致结果不准确  ACM会WA

                    m -= j[i];

                    maxF += f[i];

                }else{

                    maxF += m*a[i];

                    break;

                }

            }

            System.out.printf("%.3f",maxF);

            System.out.println();

        }

        scan.close();

    }

    static void sort(double[] a,double[] b,double[] c) {

        int len = a.length;

        int low = 0,high = len - 1;

        quickSort(a,b,c, low, high);

    }

    static void quickSort(double[] a,double[] b, double[] c,int l ,int h){

        if(l>=h){

            return;

        }

        int low = l;

        int high = h;

        double k = a[low];

        double k2 = b[low];

        double k3 = c[low];

        while(low< high){

            //

            while(high>low&&a[high]>=k){//寻找元素右边比其小的

                high --;

            }

            a[low] = a[high];//进行交换，K指向high

            b[low] = b[high];

            c[low] = c[high];

            while(low<high&&a[low]<=k){//寻找元素左边比其大的

                low++;

            }

            a[high] = a[low];//进行交换，K指向low

            b[high] = b[low];

            c[high] = c[low];

        }

        a[low] = k;//将K赋给low

        b[low] = k2;

        c[low] = k3;

        quickSort(a,b, c,l, low-1);

        quickSort(a,b, c,low+1, h);

    }

}

import java.util.*;

import java.io.*;

public class Main{

    public static void main(String[] arg){

        Scanner scan = new Scanner(new BufferedInputStream(System.in));

        int m,n;

        while((m=scan.nextInt())!=-1&&(n=scan.nextInt())!=-1){

            ArrayList<Trade> trades = new ArrayList<>(n);

            double maxF=0;

            for(int i =0 ; i != n ; i ++){

                Trade trade = new Trade();

                trade.setF(scan.nextDouble());

                trade.setJ(scan.nextDouble());

                trades.add(trade);

            }

            Collections.sort(trades);

            for(int i =0 ; i !=n ; i ++ ){

                Trade trade = trades.get(i);

                if(m>= trade.getJ() ){

                    m -= trade.getJ() ;

                    maxF += trade.getF() ;

                }else{

                    maxF += m*trade.getA();

                    break;

                }

            }

            System.out.printf("%.3f",maxF);

            System.out.println();

        }

        scan.close();

    }

    static class Trade implements Comparable<Trade> {

        private double f;//鼠食

        private double j;//猫粮

        @Override

        public int compareTo(Trade o) {

            if(getA()>o.getA()){

                return -1;

            }else if(getA()==o.getA()){

                return 0;

            }

            return 1;

        }

        public double getF() {

            return f;

        }

        public void setF(double f) {

            this.f = f;

        }

        public double getJ() {

            return j;

        }

        public void setJ(double j) {

            this.j = j;

        }

        public double getA() {//获取性价比

            return f/j;

        }

    }

}