while(1):
    try:
        m,n=map(int,input().split())
        if(m==-1 and n==-1):
            break
        s=[]
        for ii in range(n):
            a,b=map(int,input().split())
            k=a/b
            s.append((a,b,k))
        t=sorted(s,key=lambda x:-x[2])
        # print(t)
        count=0
        for bb,aa,cc in t:
            if(m>=aa):
                count+=bb
                m-=aa
            else:
                count+=(m/aa)*bb
                m=0
        print("%.3f"%count)
    except:
        break