'''
stus = []
class Student:
    def __init__(self,id,data,ave):
        self.id = id
        self.data= data
        self.ave=ave
n=int(input())
for i in range(n):
  t=list(map(int,input().split()))
  x=t[0:1]
  y=t[1:]
  y=sorted(y)
  stus.append(Student(id=x[0],data=y,ave=sum(y[1:4])/3))
stulist = sorted(stus, key=lambda zz:(-zz.ave, zz.id))
for i in range(3):
  print("%d %.3lf"%(stulist[i].id,stulist[i].ave))



    
'''  


class Hk:
  def __init__(self,price,num):
    self.price=price
    self.num=num
hks=[]

total,n=list(map(int,input().split()))
for i in range(n):
  price,num=list(map(int,input().split()))
  hks.append(Hk(price,num))

hklist = sorted(hks,key=lambda t:(t.price))
hksum=0
pricesum=0
i=0

  
while hksum+hklist[i].num<total:
  hksum=hksum+hklist[i].num
  pricesum=pricesum+hklist[i].price*hklist[i].num
  i=i+1
else:
  pricesum=pricesum+hklist[i].price*(total-hksum)
print(pricesum)
 


/**************************************************************
	Problem: 1730
	User: admin
	Language: Python
	Result: Accepted
	Time:324 ms
	Memory:34480 kb
****************************************************************/