---

n, k, w= map(int, input@).split()
clients = ［］
for - in range(k):
c, wi = map(int, input().split())
clients. append((c, wi))
clients.sort(reverse=True)
places = n* w
ans = 0
for c, wi in clients:
take = min(wi, places)
ans += take * c places -= take if places == 0:
break
print(ans)