class Cow:
  def __init__(self,price,num):
    self.price=price
    self.num=num
  def __lt__(self,other):
    return self.price<other.price or self.price==other.price and self.num>other.num
  def __str__(self):
    return "%d %d"%(self.price,self.num)
    
total,n=list(map(int,input().split()))
lst=list()
for i in range(n):
  price,num=list(map(int,input().split()))
  lst.append(Cow(price,num))

lst.sort()
num_sum=0
tqsum=0
i=0
for item in lst:
  if(total>=item.num):
    total = total-item.num
    tqsum=tqsum+item.num*item.price
  else:
    tqsum=tqsum+(total)*item.price
    total=0
    break
  
print(tqsum)

/**************************************************************
	Problem: 1940
	User: admin
	Language: Python
	Result: Accepted
	Time:991 ms
	Memory:35636 kb
****************************************************************/