##### Short Problem Definition:

The member states of the UN are planning to send *2* people to the moon. They want them to be from different countries. You will be given a list of pairs of astronaut ID’s. Each pair is made of astronauts from the same country. Determine how many pairs of astronauts from different countries they can choose from.

##### Link

##### Complexity:

time complexity is `O(N)`

space complexity is `O(1)`

##### Execution:

Keep track of _M_elements at a time. Basically a simplified prefix sum.

##### Solution:

```
#!/bin/python
import sys
def getWays(squares, d, m):
cnt = 0
q = squares[:m-1]
for ele in squares[m-1:]:
q.append(ele)
if (sum(q) == d):
cnt += 1
q.pop(0)
return cnt
n = int(raw_input().strip())
s = map(int, raw_input().strip().split(' '))
d,m = raw_input().strip().split(' ')
d,m = [int(d),int(m)]
result = getWays(s, d, m)
print(result)
```