Find missing number in sequence with python

This code snippet is designed to find the missing number in a sequence of integers with python 2.7.

I’ve tested it with several lists in python which are commented at the start of the script.

#!/usb/bin/python
#miguel.ortiz
#s=[-1, -4, -7, -10, -13, -16, -19, -25, -28]
#s=[1, 4, 7, 10, 13, 16, 19, 25, 28]
s=[1.1,2,3,4,5,6,7,8,10]
#s=[1, 3, 4, 5, 6, 7, 8, 9]
#s=[-1, 2, 5, 8, 11, 14, 17, 20,26]

diff=[]

for i,j in zip(s,s[1:]):
    diff.append(i - j)

for x,y in zip(diff,diff[1:]) :
    if not x == y :
        pointer=s[diff.index(y)]
        if pointer < 0 :
            number= s[diff.index(y) -1] + -(y)
            break
        else :
            number= s[diff.index(y) -1] + abs(y)
            break
print number

The next snippet do the same thing in less code:

#!/usb/bin/python
#s=[-1, -4, -7, -10, -13, -16, -19, -25, -28]
s=[1,2,3,4,5,6,7,8,10]

return (s[0] + t[-s]) * (len(s) + 1) / 2 - sum(s)

Or this one:

#!/usb/bin/python
#s=[-1, -4, -7, -10, -13, -16, -19, -25, -28]
s=[1,2,3,4,5,6,7,8,10]

step = int((max(s)-min(s))/len(s))
return sum(range(min(s),max(s)+1,step))-sum(s)