input =
sys.
stdin.
readline
case =
dict()
W,
N =
map(
int,
input().
split(
' '))
for
_
in
range(
N):
M,
P =
map(
int,
input().
split(
' '))
case[
P] =
M
list1 =
sorted(
list(
case.
keys()),
reverse=
True)
price =
0
for
i
in
list1:
if
W >
case[
i]:
price +=
case[
i] *
i
W -=
case[
i]
else:
price +=
W *
i
break
print(
price)