# Python: Find the number of combinations

## Python Basic - 1: Exercise-62 with Solution

Write a Python program to find the number of combinations that satisfy p + q + r + s = n where n is a given number <= 4000 and p, q, r, s are between 0 to 1000.

**Sample Solution**:

**Python Code:**

```
from collections import Counter
print("Input a positive integer: (ctrl+d to exit)")
pair_dict = Counter()
for i in range(2001):
pair_dict[i] = min(i, 2000 - i) + 1
while True:
try:
n = int(input())
ans = 0
for i in range(n + 1):
ans += pair_dict[i] * pair_dict[n - i]
print("Number of combinations of a,b,c,d:",ans)
except EOFError:
break
```

Sample Output:

Input a positive integer: (ctrl+d to exit) 252 Number of combinations of a,b,c,d: 2731135

**Flowchart:**

**Python Code Editor:**

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