Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env pypy
from __future__ import division, print_function

import os
import sys
from __builtin__ import xrange as range
from cStringIO import StringIO
from future_builtins import ascii, filter, hex, map, oct, zip
from io import IOBase
from collections import defaultdict

import __pypy__


def main():
    n = int(input())
    a = [int(i) for i in input().split(" ")]
    s = defaultdict(int)
    for i in xrange(n):
    	ans += s[a[i]-i]
        s[-i-a[i]] += 1
    print(ans)


# region fastio

BUFSIZE = 8192


class FastI(IOBase):
    def __init__(self, file):
        self._fd = file.fileno()
        self._buffer = StringIO()
        self.newlines = 0

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count("\n") + (not b)
            ptr = self._buffer.tell()
            self._buffer.seek(0, 2), self._buffer.write(b), self._buffer.seek(ptr)
        self.newlines -= 1
        return self._buffer.readline()


class FastO(IOBase):
    def __init__(self, file):
        self._fd = file.fileno()
        self._buffer = __pypy__.builders.StringBuilder()
        self.write = lambda s: self._buffer.append(s)

    def flush(self):
        os.write(self._fd, self._buffer.build())
        self._buffer = __pypy__.builders.StringBuilder()


def print(*args, **kwargs):
    sep, file = kwargs.pop("sep", " "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", "\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastI(sys.stdin), FastO(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")

# endregion

if __name__ == "__main__":
    main()


Fixed submission:
from collections import defaultdict
import sys

n = int(raw_input())
a = [int(i) for i in raw_input().split(' ')]
s = defaultdict(int)
ans = 0

for i in xrange(n):
  ans += s[a[i] - i]
  s[-i-a[i]] += 1

print ans

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env pypy
from __future__ import division, print_function

import os
import sys
from __builtin__ import xrange as range
from cStringIO import StringIO
from future_builtins import ascii, filter, hex, map, oct, zip
from io import IOBase
from collections import defaultdict

import __pypy__


def main():
    n = int(input())
    a = [int(i) for i in input().split(" ")]
    s = defaultdict(int)
    for i in xrange(n):
    	ans += s[a[i]-i]
        s[-i-a[i]] += 1
    print(ans)


# region fastio

BUFSIZE = 8192


class FastI(IOBase):
    def __init__(self, file):
        self._fd = file.fileno()
        self._buffer = StringIO()
        self.newlines = 0

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count("\n") + (not b)
            ptr = self._buffer.tell()
            self._buffer.seek(0, 2), self._buffer.write(b), self._buffer.seek(ptr)
        self.newlines -= 1
        return self._buffer.readline()


class FastO(IOBase):
    def __init__(self, file):
        self._fd = file.fileno()
        self._buffer = __pypy__.builders.StringBuilder()
        self.write = lambda s: self._buffer.append(s)

    def flush(self):
        os.write(self._fd, self._buffer.build())
        self._buffer = __pypy__.builders.StringBuilder()


def print(*args, **kwargs):
    sep, file = kwargs.pop("sep", " "), kwargs.pop("file", sys.stdout)
    at_start = True
    for x in args:
        if not at_start:
            file.write(sep)
        file.write(str(x))
        at_start = False
    file.write(kwargs.pop("end", "\n"))
    if kwargs.pop("flush", False):
        file.flush()


sys.stdin, sys.stdout = FastI(sys.stdin), FastO(sys.stdout)
input = lambda: sys.stdin.readline().rstrip("\r\n")

# endregion

if __name__ == "__main__":
    main()

Fixed submission:
from collections import defaultdict
import sys

n = int(raw_input())
a = [int(i) for i in raw_input().split(' ')]
s = defaultdict(int)
ans = 0

for i in xrange(n):
  ans += s[a[i] - i]
  s[-i-a[i]] += 1

print ans