content_beans_count, packages_count, members_count =  map(int,input().split())

family_years = [*map(int,input().split())]
family_years = sorted(family_years)
beans_total = content_beans_count * packages_count

for i in family_years:
    if beans_total >= 0:
        beans_total -= i
    else:
        print(-1)
        exit()

print(beans_total)