# Task
 Given an array `arr` and a number `n`. Call a pair of numbers from the array a `Perfect Pair` if their sum is equal to `n`.

 Find all of the perfect pairs and return the sum of their **indices**. 
 
 Note that any element of the array can only be counted in one Perfect Pair. Also if there are multiple correct answers, return the smallest one.

# Example

 For `arr = [1, 4, 2, 3, 0, 5] and n = 7`, the result should be `11`.
 
 Since the Perfect Pairs are `(4, 3)` and `(2, 5)` with indices `1 + 3 + 2 + 5 = 11`.

 For `arr = [1, 3, 2, 4] and n = 4`, the result should be `1`.
 
 Since the element at `index 0` (i.e. 1) and the element at `index 1` (i.e. 3) form the only Perfect Pair.

# Input/Output


 - `[input]` integer array `arr`

  array of non-negative integers.


 - `[input]` integer `n`

  positive integer


 - `[output]` integer

  sum of indices and 0 if no Perfect Pair exists.

```python

USER_FILENAME = "<user_code>"
import sys, json, builtins
import math, heapq, bisect, itertools, functools, collections, statistics, fractions, decimal, operator, array, random, re, string, os, pathlib
try:
    import resource as _res
except Exception:
    _res = None
from collections import Counter, defaultdict, deque, OrderedDict
from functools import lru_cache, reduce
from itertools import accumulate, permutations, combinations, product
from math import gcd, ceil, floor, sqrt
from decimal import Decimal
try:
    from typing import *
except Exception:
    pass
try:
    sys.setrecursionlimit(1_000_000)
except Exception:
    pass
try:
    import site as _site
    try:
        _sys_sites = _site.getsitepackages() if hasattr(_site, "getsitepackages") else []
        for _p in _sys_sites:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
    try:
        _usp = _site.getusersitepackages()
        if isinstance(_usp, str):
            _usp = [_usp]
        for _p in _usp:
            if _p and _p not in sys.path:
                sys.path.append(_p)
    except Exception:
        pass
except Exception:
    pass

def _disabled_input(*args, **kwargs):
    raise RuntimeError("input() is disabled in callable mode")
builtins.input = _disabled_input

ns = {"__name__": "__judge__"}  # prevent if __name__ == "__main__" blocks
for _k, _v in list(globals().items()):
    if not _k.startswith("_"):
        ns[_k] = _v
_CODE = 'from collections import defaultdict\n\ndef pairwise(arr,n):\n    ixs=defaultdict(list)\n    for i,e in enumerate(arr): ixs[e].append(i)\n    h=n/2\n    t=sum(x+y for a,ai in (p for p in ixs.items() if p[0]<h) for x,y in zip(ai,ixs.get(n-a,[])))\n    if n%2: return t\n    hi=ixs.get(int(h),[])\n    return t+sum(hi[:len(hi)//2*2])'
_co = compile(_CODE, USER_FILENAME, "exec")
exec(_co, ns, ns)

_target = None
_fn_name = 'pairwise'
if _fn_name:
    if "Solution" in ns:
        try:
            _obj = ns["Solution"]()
            if hasattr(_obj, _fn_name):
                _target = getattr(_obj, _fn_name)
        except Exception:
            _target = None
    if _target is None and _fn_name in ns and callable(ns[_fn_name]):
        _target = ns[_fn_name]
# Fallback: if there's exactly one user-defined function, use it
if _target is None:
    _cands = [v for k, v in ns.items() if callable(v) and getattr(v, "__module__", "") in ("__judge__", "__main__")]
    _user_funcs = [c for c in _cands if getattr(c, "__name__", "").startswith("_") is False]
    if len(_user_funcs) == 1:
        _target = _user_funcs[0]
if _target is None:
    raise RuntimeError(f"Could not resolve callable '{_fn_name}'")

_raw = sys.stdin.read().strip()
if _raw:
    _parsed = json.loads(_raw)
    if isinstance(_parsed, dict):
        _kwargs = _parsed.get("kwargs", {})
        _args = _parsed.get("args", [])
    else:
        _args, _kwargs = _parsed, {}
else:
    _args, _kwargs = [], {}
if not isinstance(_args, (list, tuple)):
    _args = [_args]
_res = _target(*_args, **_kwargs)
try:
    sys.stdout.write(repr(_res) + "\n")
except Exception:
    pass

```

Measured memory_bytes: 8270