// The main code is at the very bottom.

#[allow(unused_imports)]
use {
  lib::byte::ByteChar,
  std::cell::{Cell, RefCell},
  std::cmp::{
    self,
    Ordering::{self, *},
    Reverse,
  },
  std::collections::*,
  std::convert::identity,
  std::fmt::{self, Debug, Display, Formatter},
  std::io::prelude::*,
  std::iter::{self, FromIterator},
  std::marker::PhantomData,
  std::mem,
  std::num::Wrapping,
  std::ops::{Range, RangeFrom, RangeInclusive, RangeTo, RangeToInclusive},
  std::process,
  std::rc::Rc,
  std::thread,
  std::time::{Duration, Instant},
  std::{char, f32, f64, i128, i16, i32, i64, i8, isize, str, u128, u16, u32, u64, u8, usize},
};

#[allow(unused_imports)]
#[macro_use]
pub mod lib {
  pub mod byte {
    pub use self::byte_char::*;

    mod byte_char {
      use std::error::Error;
      use std::fmt::{self, Debug, Display, Formatter};
      use std::str::FromStr;

      #[derive(Clone, Copy, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
      #[repr(transparent)]
      pub struct ByteChar(pub u8);

      impl Debug for ByteChar {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          write!(f, "b'{}'", self.0 as char)
        }
      }

      impl Display for ByteChar {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          write!(f, "{}", self.0 as char)
        }
      }

      impl FromStr for ByteChar {
        type Err = ParseByteCharError;

        fn from_str(s: &str) -> Result<ByteChar, ParseByteCharError> {
          match s.as_bytes().len() {
            1 => Ok(ByteChar(s.as_bytes()[0])),
            0 => Err(ParseByteCharErrorKind::EmptyStr.into()),
            _ => Err(ParseByteCharErrorKind::TooManyBytes.into()),
          }
        }
      }

      #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
      pub struct ParseByteCharError {
        kind: ParseByteCharErrorKind,
      }

      impl Display for ParseByteCharError {
        fn fmt(&self, f: &mut Formatter) -> fmt::Result {
          f.write_str(match self.kind {
            ParseByteCharErrorKind::EmptyStr => "empty string",
            ParseByteCharErrorKind::TooManyBytes => "too many bytes",
          })
        }
      }

      impl Error for ParseByteCharError {}

      #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
      enum ParseByteCharErrorKind {
        EmptyStr,
        TooManyBytes,
      }

      impl From<ParseByteCharErrorKind> for ParseByteCharError {
        fn from(kind: ParseByteCharErrorKind) -> ParseByteCharError {
          ParseByteCharError { kind }
        }
      }
    }
  }

  pub mod io {
    pub use self::scanner::*;

    mod scanner {
      use std::io::{self, BufRead};
      use std::iter;
      use std::str::FromStr;

      #[derive(Debug)]
      pub struct Scanner<R> {
        reader: R,
        buf: String,
        pos: usize,
      }

      impl<R: BufRead> Scanner<R> {
        pub fn new(reader: R) -> Self {
          Scanner {
            reader,
            buf: String::new(),
            pos: 0,
          }
        }

        pub fn next(&mut self) -> io::Result<&str> {
          let start = loop {
            match self.rest().find(|c| c != ' ') {
              Some(i) => break i,
              None => self.fill_buf()?,
            }
          };
          self.pos += start;
          let len = self.rest().find(' ').unwrap_or(self.rest().len());
          let s = &self.buf[self.pos..][..len]; // self.rest()[..len]
          self.pos += len;
          Ok(s)
        }

        pub fn parse_next<T>(&mut self) -> io::Result<Result<T, T::Err>>
        where
          T: FromStr,
        {
          Ok(self.next()?.parse())
        }

        pub fn parse_next_n<T>(&mut self, n: usize) -> io::Result<Result<Vec<T>, T::Err>>
        where
          T: FromStr,
        {
          iter::repeat_with(|| self.parse_next()).take(n).collect()
        }

        pub fn map_next_bytes<T, F>(&mut self, mut f: F) -> io::Result<Vec<T>>
        where
          F: FnMut(u8) -> T,
        {
          Ok(self.next()?.bytes().map(&mut f).collect())
        }

        pub fn map_next_bytes_n<T, F>(&mut self, n: usize, mut f: F) -> io::Result<Vec<Vec<T>>>
        where
          F: FnMut(u8) -> T,
        {
          iter::repeat_with(|| self.map_next_bytes(&mut f))
            .take(n)
            .collect()
        }

        fn rest(&self) -> &str {
          &self.buf[self.pos..]
        }

        fn fill_buf(&mut self) -> io::Result<()> {
          self.buf.clear();
          self.pos = 0;
          let read = self.reader.read_line(&mut self.buf)?;
          if read == 0 {
            return Err(io::ErrorKind::UnexpectedEof.into());
          }
          if *self.buf.as_bytes().last().unwrap() == b'\n' {
            self.buf.pop();
          }
          Ok(())
        }
      }
    }
  }
}

#[allow(unused_macros)]
macro_rules! eprint {
  ($($arg:tt)*) => {
    if cfg!(debug_assertions) {
      std::eprint!($($arg)*)
    }
  };
}
#[allow(unused_macros)]
macro_rules! eprintln {
  ($($arg:tt)*) => {
    if cfg!(debug_assertions) {
      std::eprintln!($($arg)*)
    }
  };
}
#[allow(unused_macros)]
macro_rules! dbg {
  ($($arg:tt)*) => {
    if cfg!(debug_assertions) {
      std::dbg!($($arg)*)
    } else {
      ($($arg)*)
    }
  };
}

const CUSTOM_STACK_SIZE_MIB: Option<usize> = Some(128);
const INTERACTIVE: bool = false;

fn main() -> std::io::Result<()> {
  match CUSTOM_STACK_SIZE_MIB {
    Some(stack_size_mib) => std::thread::Builder::new()
      .name("run_solver".to_owned())
      .stack_size(stack_size_mib * 1024 * 1024)
      .spawn(run_solver)?
      .join()
      .unwrap(),
    None => run_solver(),
  }
}

fn run_solver() -> std::io::Result<()> {
  let stdin = std::io::stdin();
  let reader = stdin.lock();
  let stdout = std::io::stdout();
  let writer = stdout.lock();
  macro_rules! with_wrapper {
    ($($wrapper:expr)?) => {{
      let mut writer = $($wrapper)?(writer);
      solve(reader, &mut writer)?;
      writer.flush()
    }};
  }
  if cfg!(debug_assertions) {
    with_wrapper!()
  } else if INTERACTIVE {
    with_wrapper!(std::io::LineWriter::new)
  } else {
    with_wrapper!(std::io::BufWriter::new)
  }
}

fn solve<R, W>(reader: R, mut writer: W) -> std::io::Result<()>
where
  R: BufRead,
  W: Write,
{
  let mut _scanner = lib::io::Scanner::new(reader);
  #[allow(unused_macros)]
  macro_rules! scan {
    ($T:ty) => {
      _scanner.parse_next::<$T>()?.unwrap()
    };
    ($($T:ty),+) => {
      ($(scan!($T)),+)
    };
    ($T:ty; $n:expr) => {
      _scanner.parse_next_n::<$T>($n)?.unwrap()
    };
    ($($T:ty),+; $n:expr) => {
      iter::repeat_with(|| -> std::io::Result<_> { Ok(($(scan!($T)),+)) })
        .take($n)
        .collect::<std::io::Result<Vec<_>>>()?
    };
  }
  #[allow(unused_macros)]
  macro_rules! scan_bytes_map {
    ($f:expr) => {
      _scanner.map_next_bytes($f)?
    };
    ($f:expr; $n:expr) => {
      _scanner.map_next_bytes_n($n, $f)?
    };
  }
  #[allow(unused_macros)]
  macro_rules! print {
    ($($arg:tt)*) => {
      write!(writer, $($arg)*)?
    };
  }
  #[allow(unused_macros)]
  macro_rules! println {
    ($($arg:tt)*) => {
      writeln!(writer, $($arg)*)?
    };
  }
  {
    // Thank you!
    // ref. https://github.com/noshi91/n91lib_rs
    #[allow(unused)]
    mod n91lib {
      pub mod data_structure {
        mod bit_vector {
          use super::super::other::bit::access;
          use super::super::other::bit::rank;
          use super::super::other::bit::select;
          use super::super::other::bit::WORD;
          use std::iter::FromIterator;
          use std::iter::IntoIterator;
          use std::iter::Iterator;

          pub struct BitVector {
            data: Box<[Node]>,
          }

          struct Node {
            bit: usize,
            sum: usize,
          }

          impl BitVector {
            pub fn access(&self, index: usize) -> bool {
              access(self.data[index / WORD].bit, index % WORD)
            }

            pub fn rank0(&self, end: usize) -> usize {
              end - self.rank1(end)
            }

            pub fn rank1(&self, end: usize) -> usize {
              let t = &self.data[end / WORD];
              t.sum + rank(t.bit, end % WORD)
            }

            pub fn select0(&self, k: usize) -> usize {
              let (mut st, mut en) = (0, self.data.len());
              while en - st != 1 {
                let mid = (st + en) / 2;
                if mid * WORD - self.data[mid].sum <= k {
                  st = mid;
                } else {
                  en = mid;
                }
              }
              let rem = k - (st * WORD - self.data[st].sum);
              st * WORD + select(!self.data[st].bit, rem)
            }

            pub fn select1(&self, k: usize) -> usize {
              let (mut st, mut en) = (0, self.data.len());
              while en - st != 1 {
                let mid = (st + en) / 2;
                if self.data[mid].sum <= k {
                  st = mid;
                } else {
                  en = mid;
                }
              }
              let rem = k - self.data[st].sum;
              st * WORD + select(self.data[st].bit, rem)
            }
          }

          impl FromIterator<bool> for BitVector {
            fn from_iter<T: IntoIterator<Item = bool>>(iter: T) -> Self {
              let mut iter = iter.into_iter();
              let mut v = Vec::new();
              let mut sum = 0;
              'base: loop {
                let mut bit = 0;
                for i in 0..WORD {
                  match iter.next() {
                    Some(v) => {
                      if v {
                        bit |= 1 << i;
                      }
                    }
                    None => {
                      v.push(Node { bit: bit, sum: sum });
                      break 'base;
                    }
                  }
                }
                v.push(Node { bit: bit, sum: sum });
                sum += bit.count_ones() as usize;
              }
              Self {
                data: v.into_boxed_slice(),
              }
            }
          }
        }

        mod wavelet_matrix {
          use super::super::data_structure::BitVector;
          use super::super::other::bit::access;
          use std::ops::Range;

          pub struct WaveletMatrix {
            data: Box<[(usize, BitVector)]>,
          }

          impl WaveletMatrix {
            pub fn new(bitlen: usize, mut seq: Vec<usize>) -> Self {
              let len = seq.len();
              let mut data = Vec::new();
              for l in (0..bitlen).rev() {
                let v = seq.iter().map(|&x| access(x, l)).collect::<BitVector>();
                data.push((v.rank0(len), v));
                let zeros = seq.iter().filter(|&&x| !access(x, l)).cloned();
                let ones = seq.iter().filter(|&&x| access(x, l)).cloned();
                seq = zeros.chain(ones).collect();
              }
              Self {
                data: data
                  .into_iter()
                  .rev()
                  .collect::<Vec<_>>()
                  .into_boxed_slice(),
              }
            }

            pub fn access(&self, mut index: usize) -> usize {
              let mut ret = 0;
              for (l, &(z, ref v)) in self.base_iter().rev() {
                if !v.access(index) {
                  index = v.rank0(index);
                } else {
                  ret |= 1 << l;
                  index = z + v.rank1(index);
                }
              }
              ret
            }

            pub fn rank(&self, value: usize, mut range: Range<usize>) -> usize {
              for (l, &(z, ref v)) in self.base_iter().rev() {
                if !access(value, l) {
                  range.start = v.rank0(range.start);
                  range.end = v.rank0(range.end);
                } else {
                  range.start = z + v.rank1(range.start);
                  range.end = z + v.rank1(range.end);
                }
              }
              range.end - range.start
            }

            pub fn select(&self, value: usize, k: usize) -> usize {
              let mut index = 0;
              for (l, &(z, ref v)) in self.base_iter().rev() {
                if !access(value, l) {
                  index = v.rank0(index);
                } else {
                  index = z + v.rank1(index);
                }
              }
              index += k;
              for (_, &(z, ref v)) in self.base_iter() {
                if index < z {
                  index = v.select0(index);
                } else {
                  index = v.select1(index - z);
                }
              }
              index
            }

            pub fn count(&self, idxrng: Range<usize>, valrng: Range<usize>) -> usize {
              self.count_to(idxrng.clone(), valrng.end) - self.count_to(idxrng, valrng.start)
            }

            pub fn quantile(&self, mut range: Range<usize>, mut k: usize) -> usize {
              let mut ret = 0;
              for (l, &(z, ref v)) in self.base_iter().rev() {
                let zeros = v.rank0(range.end) - v.rank0(range.start);
                if zeros > k {
                  range.start = v.rank0(range.start);
                  range.end = v.rank0(range.end);
                } else {
                  k -= zeros;
                  ret |= 1 << l;
                  range.start = z + v.rank1(range.start);
                  range.end = z + v.rank1(range.end);
                }
              }
              ret
            }

            fn count_to(&self, mut range: Range<usize>, val: usize) -> usize {
              let mut ret = 0;
              for (l, &(z, ref v)) in self.base_iter().rev() {
                if !access(val, l) {
                  range.start = v.rank0(range.start);
                  range.end = v.rank0(range.end);
                } else {
                  ret += v.rank0(range.end) - v.rank0(range.start);
                  range.start = z + v.rank1(range.start);
                  range.end = z + v.rank1(range.end);
                }
              }
              ret
            }

            fn base_iter(&self) -> impl DoubleEndedIterator<Item = (usize, &(usize, BitVector))> {
              self.data.iter().enumerate()
            }
          }
        }

        pub use self::bit_vector::BitVector;
        pub use self::wavelet_matrix::WaveletMatrix;
      }

      pub mod other {
        pub mod bit {
          pub const WORD: usize = (0 as usize).count_zeros() as usize;

          pub fn access(bit: usize, index: usize) -> bool {
            bit & 1 << index != 0
          }

          pub fn rank(bit: usize, end: usize) -> usize {
            (bit & !(!0 << end)).count_ones() as usize
          }

          pub fn select(bit: usize, k: usize) -> usize {
            let (mut st, mut en) = (0, WORD);
            while en - st != 1 {
              let mid = (st + en) / 2;
              if rank(bit, mid) <= k {
                st = mid;
              } else {
                en = mid;
              }
            }
            st
          }
        }
      }
    }

    type Wm = n91lib::data_structure::WaveletMatrix;

    let n = scan!(usize);
    let a = scan!(usize; n);

    let b = (0..n).map(|i| (n - i - 1) + a[i]).collect::<Vec<_>>();
    let wm = Wm::new(63, b);
    let mut cnt = 0;
    for i in 0..n {
      if (n - 1) >= (i + a[i]) {
        let val = (n - 1) - (i + a[i]);
        cnt += wm.rank(val, i + 1..n);
      }
    }
    println!("{}", cnt);
  }
  Ok(())
}


Measured memory_bytes: 11760