n,m=map(int,input().split())
danjian=[]
shuliang={}
sum=0
qsum=0
for i in range(m):
    x,y=map(int,input().split())
    danjian=danjian+[x]
    shuliang[x]=y
danjian.sort()
for i in range(m):
    if sum<n:
        sum=sum+shuliang[danjian[i]]
        qsum=qsum+danjian[i]*shuliang[danjian[i]]
    elif sum>n:
        if (sum>=n):
            cha=sum-n
            sum=sum-cha
            qsum=qsum-danjian[i-1]*cha
        elif sum<n:
            qsum=qsum+danjian[i-1]
            break
print(qsum)