Adding up a list

[fullmoon84]

if we allow user to key in all the datas that they have, and we will help them sum up all, how to do?? below are program i do but it did.t work.

run=0
while run<>999:

#insert number of datas you have
n=raw_input("insert n:")
n=int(n)
x=int(n)

for x in range (1,n+1):
if x == n+2:
break

print"insert value of a:\n"

a=raw_input("insert a:\t")
a=float(a)

# X= total value of a
print"\nX=total value of a"
def add(x) : return a+a
x=int(a+a)
print"X=%d+%d+%d=" % (a,a),a+a


can anyone help me?? thank you