[백준] 1049

n, m = map(int, input().split())

s = []
k = []
for i in range(m):
  a, b = map(int, input().split())
  s.append(a)
  k.append(b)

best1 = min(s)
best2 = min(k)
best = min([best1, best2*6])
cost = 0
while n >= 6:
  cost += best
  n -= 6

if best < n*best2:
  cost += best
else:
  cost += n*best2

print(cost)