#include <bits/stdc++.h>
using namespace std;

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

template <class T> struct is_iterator {
	template <class U, typename enable_if<!is_convertible<U, const char*>::value, int>::type = 0>
		constexpr static auto has_indirection(int) -> decltype(*declval<U>(), bool()) { return true; }
	template <class> constexpr static bool has_indirection(long) { return false; }
	constexpr static bool value = has_indirection<T>(0);
};

using uint = unsigned int;
// Buffer size should be 2^12 or 2^13 for optimal performance with files.
const uint BUFFER_SIZE = 1 << 12;
// Maximum possible length of a string representing primitive type
// assuming we won't encounter huge double values.
const uint MAX_LENGTH = 1 << 7;

namespace Detail {
	struct Width { uint value; };
	struct Fill { char value; };
	struct Base { uint value; };
	struct Precision { uint value; };
	struct Delimiter { const char* value; };
}  // namespace Detail

Detail::Width setWidth(uint value = 0) { return {value}; }
Detail::Fill setFill(char value = ' ') { return {value}; }
Detail::Base setBase(uint value = 10) { assert(2 <= value && value <= 36); return {value}; }
Detail::Precision setPrecision(uint value = 9) { assert(value < MAX_LENGTH); return {value}; }
Detail::Delimiter setDelimiter(const char* value = " ") { return {value}; }

/******************************* input classes ********************************/
class InputDevice {
	protected:
		const char* head;
		const char* tail;

		InputDevice(const char* head, const char* tail) : head(head), tail(tail), base(setBase().value) {}

		virtual void fillInput() = 0;

		inline char nextChar() {
			if (__builtin_expect(head >= tail, false)) fillInput();
			return *head++;
		}

		template <class I> int readUnsignedIntGeneral(I& arg, char c) {
			I value = 0;
			int length = 0;
			for (;; ++length, c = nextChar()) {
				if (isDigit(c)) c -= '0';
				else if (isUpper(c)) c -= 'A' - 10;
				else if (isLower(c)) c -= 'a' - 10;
				else c = base;
				if (c >= base) break;
				value = base * value + c;
			}
			arg = value;
			return --head, length;
		}

		template <class I> inline int readUnsignedInt(I& arg, char c) {
			if (__builtin_expect(base > 10, false)) return readUnsignedIntGeneral(arg, c);
			I value = 0;
			int length = 0;
			for (; static_cast<unsigned char>(c - '0') < base; ++length, c = nextChar())
				value = base * value + c - '0';
			arg = value;
			return --head, length;
		}

		template <class I> inline bool readSignedInt(I& arg, char c) {
			bool negative = c == '-';
			if (negative) c = nextChar();
			typename make_unsigned<I>::type unsignedArg;
			if (readUnsignedInt(unsignedArg, c) == 0) return false;
			arg = negative ? ~static_cast<I>(unsignedArg - 1) : static_cast<I>(unsignedArg);
			return true;
		}

		template <class F> bool readFloatingPoint(F& arg, char c) {
			bool negative = c == '-';
			if (negative) c = nextChar();
			unsigned long long integerPart;
			if (readUnsignedInt(integerPart, c) == 0) return false;
			arg = static_cast<F>(integerPart);
			if (nextChar() == '.') {
				unsigned long long fractionalPart = 0;
				int fractionalLength = readUnsignedInt(fractionalPart, nextChar());
				if (fractionalLength > 0) {
					unsigned long long basePower = 1;
					for (; fractionalLength; --fractionalLength) basePower *= base;
					arg += static_cast<F>(fractionalPart) / basePower;
				}
			} else --head;
			if (negative) arg = -arg;
			return true;
		}

	public:
		uint base;

		InputDevice(InputDevice const&) = delete;
		InputDevice& operator = (InputDevice const&) = delete;

		static inline bool isSpace(char c) { return static_cast<unsigned char>(c - '\t') < 5 || c == ' '; }
		static inline bool isDigit(char c) { return static_cast<unsigned char>(c - '0') < 10; }
		static inline bool isUpper(char c) { return static_cast<unsigned char>(c - 'A') < 26; }
		static inline bool isLower(char c) { return static_cast<unsigned char>(c - 'a') < 26; }
		static inline bool isOneOf(char c, const char* str) { return strchr(str, c) != nullptr; }

		void putBack() { --head; }  // can be called only once directly after successfully reading a character

		inline bool readChar(char& arg) {
			if (__builtin_expect(head >= tail, false)) {
				fillInput();
				if (__builtin_expect(head >= tail, false)) return arg = '\0', false;
			}
			return arg = *head++, true;
		}

		template <class UnaryPredicate>
			inline char skipCharacters(UnaryPredicate isSkipped) {
				char c;
				do { c = nextChar(); } while (isSkipped(c));
				return c;
			}
		inline char skipCharacters() { return skipCharacters(isSpace); }

		template <class UnaryPredicate>
			inline int readString(char* arg, int limit, UnaryPredicate isTerminator) {
				skipCharacters(isTerminator);
				// put back first non-skipped character, reserve space for null character
				int charsRead = 0;
				for (--head, --limit; head < tail; fillInput()) {
					ptrdiff_t chunkSize = find_if(head, min(tail, head + limit - charsRead), isTerminator) - head;
					arg = copy_n(head, chunkSize, arg);
					head += chunkSize;
					charsRead += chunkSize;
					if (chunkSize == 0 || head < tail) break;
				}
				return *arg = '\0', charsRead;
			}

		inline int readString(char* arg, int limit, const char* terminators) {
			if (!*terminators) return readString(arg, limit, InputDevice::isSpace);
			return readString(arg, limit, [terminators](char c) { return InputDevice::isOneOf(c, terminators); });
		}

		// property setters
		inline bool read(Detail::Base newBase) { base = newBase.value; return true; }
		// primitive types
		inline bool read() { return true; }
		inline bool read(char& arg) { return readChar(arg); }
		template <class I> inline typename enable_if<is_integral<I>::value && is_unsigned<I>::value,
				 bool>::type read(I& arg) { return readUnsignedInt(arg, skipCharacters()) > 0; }
		template <class I> inline typename enable_if<is_integral<I>::value && is_signed<I>::value,
				 bool>::type read(I& arg) { return readSignedInt(arg, skipCharacters()); }
		template <class F> inline typename enable_if<is_floating_point<F>::value,
				 bool>::type read(F& arg) { return readFloatingPoint(arg, skipCharacters()); }
		// characters skip
		inline bool read(const char& arg) { skipCharacters([arg](char c) { return arg != c; }); return true; }
		inline bool read(const char* arg) {
			if (*arg) skipCharacters([arg](char c) { return InputDevice::isOneOf(c, arg); });
			else skipCharacters();
			return putBack(), true;
		}
		inline bool read(bool (*isSkipped)(char)) { skipCharacters(isSkipped); putBack(); return true; }
		// strings
		template <class I, class Terminator, class... Ts> inline typename enable_if<is_integral<I>::value,
				 bool>::type read(char* arg, I limit, Terminator terminator, Ts&&... args) {
					 readString(arg, static_cast<int>(limit), terminator);
					 return read(forward<Ts>(args)...);
				 }
		template <class I> inline typename enable_if<is_integral<I>::value,
				 bool>::type read(char* arg, I limit) { return read(arg, limit, ""); }
		template <class... Ts>
			inline bool read(char* first, char* last, Ts&&... args) {
				return read(first, static_cast<int>(last - first), forward<Ts>(args)...);
			}
		template <int N, class... Ts>
			inline bool read(char (&arg)[N], Ts&&... args) { return read(static_cast<char*>(arg), N, forward<Ts>(args)...); }
		template <class Terminator, class... Ts>
			inline bool read(string& arg, Terminator terminator, Ts&&... args) {
				for (int length = 16, last = 0;; last += length, length <<= 1) {
					arg.resize(last + length);
					int charsRead = readString(&arg[last], length + 1, terminator);
					if (charsRead < length) {
						arg.resize(last + charsRead);
						return read(forward<Ts>(args)...);
					}
				}
			}
		inline bool read(string& arg) { return read(arg, ""); }
		// complex types and ranges
		template <class T1, class T2>
			inline bool read(pair<T1, T2>& arg) { return read(arg.first, arg.second); }
		template <class T>
			inline bool read(complex<T>& arg) {
				T real, imag;
				if (!read(real, imag)) return false;
				arg.real(real), arg.imag(imag);
				return true;
			}
		template <class T>
			inline bool read(vector<T>& arg) {
				uint n;
				if (!read(n)) return false;
				arg.resize(n);
				return read(arg.begin(), arg.end());
			}
		template <class Iterator, class... Ts> inline typename enable_if<is_iterator<Iterator>::value,
				 bool>::type read(Iterator first, Iterator last, Ts&&... args) {
					 for (; first != last; ++first) if (!read(*first)) return false;
					 return read(forward<Ts>(args)...);
				 }
		template <class Iterator, class I, class... Ts>
			inline typename enable_if<is_iterator<Iterator>::value && is_integral<I>::value,
				   bool>::type read(Iterator first, I count, Ts&&... args) { return read(first, first + count, forward<Ts>(args)...); }
		// generic forwarding
		template <class T>
			inline auto read(T& arg) -> decltype(arg.read(*this)) { return arg.read(*this); }
		template <class T0, class T1, class... Ts>
			inline typename enable_if<!is_iterator<T0>::value && !is_convertible<T0, char*>::value,
				   bool>::type read(T0&& arg0, T1&& arg1, Ts&&... args) {
					   return read(forward<T0>(arg0)) && read(forward<T1>(arg1), forward<Ts>(args)...);
				   }
};

class InputFile : public InputDevice {
	FILE* file;
	bool lineBuffered;
	bool owner;
	char buffer[BUFFER_SIZE];

	void fillInput() override {
		head = buffer;
		*buffer = '\0';
		if (__builtin_expect(!lineBuffered, true)) {
			tail = head + fread(buffer, 1, BUFFER_SIZE, file);
		} else {
			tail = head;
			if (fgets(buffer, BUFFER_SIZE, file)) while (*tail) ++tail;
		}
	}

	public:
	InputFile(FILE* file = stdin, bool lineBuffered = true, bool takeOwnership = false)
		: InputDevice(buffer, buffer) , file(file), lineBuffered(lineBuffered), owner(takeOwnership) {}
	InputFile(const char* fileName) : InputFile(fopen(fileName, "r"), false, true) {}
	~InputFile() { if (owner) fclose(file); }
};

// Picks up data appended to the string but doesn't handle reallocation.
class InputString : public InputDevice {
	void fillInput() override { while (*tail) ++tail; }

	public:
	InputString(const string& s) : InputDevice(s.data(), s.data() + s.size()) {}
	InputString(const char* s) : InputDevice(s, s + strlen(s)) {}
};

class OutputDevice {
	protected:
		char buffer[BUFFER_SIZE + MAX_LENGTH];
		char* output;
		char* end;
		bool separate;

		OutputDevice() : output(buffer), end(buffer + BUFFER_SIZE + MAX_LENGTH), separate(false)
						 , width(setWidth().value), fill(setFill().value), base(setBase().value), precision(setPrecision().value)
									 , delimiter(setDelimiter().value) { computeBasePower(); }

		virtual void writeToDevice(uint count) = 0;

		inline void flushMaybe() {
			if (__builtin_expect(output >= buffer + BUFFER_SIZE, false)) {
				writeToDevice(BUFFER_SIZE);
				output = copy(buffer + BUFFER_SIZE, output, buffer);
			}
		}

		void computeBasePower() {
			basePower = 1;
			for (uint i = 0; i < precision; ++i) basePower *= base;
		}

		template <class I> inline char* writeUnsignedInt(I arg, char* last) {
			if (__builtin_expect(arg == 0, false)) *--last = '0';
			if (__builtin_expect(base == 10, true)) {
				for (; arg; arg /= 10) *--last = '0' + arg % 10;
			} else for (; arg; arg /= base) {
				I digit = arg % base;
				*--last = digit < 10 ? '0' + digit : 'A' - 10 + digit;
			}
			return last;
		}

		template <class I> inline char* writeSignedInt(I arg, char* last) {
			auto unsignedArg = static_cast<typename make_unsigned<I>::type>(arg);
			if (arg < 0) {
				last = writeUnsignedInt(~unsignedArg + 1, last);
				*--last = '-';
				return last;
			}
			return writeUnsignedInt(unsignedArg, last);
		}

		template <class F> char* writeFloatingPoint(F arg, char* last) {
			bool negative = signbit(arg);
			if (negative) arg = -arg;
			if (isnan(arg)) for (int i = 0; i < 3; ++i) *--last = i["NaN"];
			else if (isinf(arg)) for (int i = 0; i < 3; ++i) *--last = i["fnI"];
			else {
				auto integerPart = static_cast<unsigned long long>(arg);
				auto fractionalPart = static_cast<unsigned long long>((arg - integerPart) * basePower + F(0.5));
				if (fractionalPart >= basePower) ++integerPart, fractionalPart = 0;
				char* point = last - precision;
				if (precision > 0) {
					::fill(point, writeUnsignedInt(fractionalPart, last), '0');
					*--point = '.';
				}
				last = writeUnsignedInt(integerPart, point);
			}
			if (negative) *--last = '-';
			return last;
		}

		inline int writeT(char* first) {
			int delimiterLenght = separate ? writeDelimiter() : 0;
			separate = true;
			uint charsWritten = static_cast<uint>(end - first);
			if (__builtin_expect(charsWritten < width, false))
				charsWritten += writeFill(width - charsWritten);
			output = copy(first, end, output);
			flushMaybe();
			return delimiterLenght + static_cast<int>(charsWritten);
		}

		inline int writeFill(uint count) {
			int charsWritten = static_cast<int>(count);
			if (__builtin_expect(output + count + MAX_LENGTH < end, true)) {
				if (count == 1) *output++ = fill;
				else output = fill_n(output, count, fill);
			} else for (uint chunkSize = static_cast<uint>(buffer + BUFFER_SIZE - output);; chunkSize = BUFFER_SIZE) {
				if (chunkSize > count) chunkSize = count;
				output = fill_n(output, chunkSize, fill);
				flushMaybe();
				if ((count -= chunkSize) == 0) break;
			}
			return charsWritten;
		}

	public:
		uint width;
		char fill;
		uint base;
		uint precision;
		unsigned long long basePower;
		string delimiter;

		OutputDevice(OutputDevice const&) = delete;
		OutputDevice& operator = (OutputDevice const&) = delete;
		virtual ~OutputDevice() {};

		inline int writeChar(char arg) { separate = false; *output++ = arg; flushMaybe(); return 1; }

		inline int writeString(const char* arg, size_t length, bool checkWidth = true) {
			separate = false;
			uint count = static_cast<uint>(length);
			int charsWritten = static_cast<int>(count) + (checkWidth && count < width ? writeFill(width - count) : 0);
			if (__builtin_expect(output + count + MAX_LENGTH < end, true)) {
				if (count == 1) *output++ = *arg;
				else output = copy_n(arg, count, output);
			} else for (uint chunkSize = static_cast<uint>(buffer + BUFFER_SIZE - output);; chunkSize = BUFFER_SIZE) {
				if (chunkSize > count) chunkSize = count;
				output = copy_n(arg, chunkSize, output);
				flushMaybe();
				if ((count -= chunkSize) == 0) break;
				arg += chunkSize;
			}
			return charsWritten;
		}

		inline int writeDelimiter() { return writeString(delimiter.c_str(), delimiter.size(), false); }

		inline void flush() {
			writeToDevice(static_cast<uint>(output - buffer));
			output = buffer;
		}

		// property setters
		inline int write(Detail::Width newWidth) { width = newWidth.value; return 0; }
		inline int write(Detail::Fill newFill) { fill = newFill.value; return 0; }
		inline int write(Detail::Base newBase) { base = newBase.value; computeBasePower(); return 0; }
		inline int write(Detail::Precision newPrecision) {
			precision = newPrecision.value; computeBasePower(); return 0;
		}
		inline int write(Detail::Delimiter newDelimiter) { delimiter = newDelimiter.value; return 0; }
		// primitive types
		inline int write() { return 0; }
		inline int write(char arg) { return writeChar(arg); }
		template <class I> inline typename enable_if<is_integral<I>::value && is_unsigned<I>::value,
				 int>::type write(I arg) { return writeT(writeUnsignedInt(arg, end)); }
		template <class I> inline typename enable_if<is_integral<I>::value && is_signed<I>::value,
				 int>::type write(I arg) { return writeT(writeSignedInt(arg, end)); }
		template <class F> inline typename enable_if<is_floating_point<F>::value,
				 int>::type write(F arg) { return writeT(writeFloatingPoint(arg, end)); }
		// complex types
		inline int write(const char* arg) { return writeString(arg, strlen(arg)); }
		template <int N>
			inline int write(char (&arg)[N]) { return writeString(arg, strlen(arg)); }
		inline int write(const string& arg) { return writeString(arg.c_str(), arg.size()); }
		template <class T1, class T2>
			inline int write(const pair<T1, T2>& arg) {
				int charsWritten = write(arg.first);
				charsWritten += writeDelimiter();
				return charsWritten + write(arg.second);
			}
		template <class T>
			inline int write(const complex<T>& arg) { return write(real(arg), imag(arg)); }
		// ranges
		template <class Iterator, class... Ts> inline typename enable_if<is_iterator<Iterator>::value,
				 int>::type write(Iterator first, Iterator last, Ts&&... args) {
					 int charsWritten = 0;
					 for (; first != last; charsWritten += ++first == last ? 0 : writeDelimiter()) charsWritten += write(*first);
					 return charsWritten + write(forward<Ts>(args)...);
				 }
		template <class Iterator, class I, class... Ts>
			inline typename enable_if<is_iterator<Iterator>::value && is_integral<I>::value,
				   int>::type write(Iterator first, I count, Ts&&... args) { return write(first, first + count, forward<Ts>(args)...); }
		// generic forwarding
		template <class T>
			inline auto write(const T& arg) -> decltype(arg.write(*this)) { return arg.write(*this); }
		template <class T0, class T1, class... Ts> inline typename enable_if<!is_iterator<T0>::value,
				 int>::type write(T0&& arg0, T1&& arg1, Ts&&... args) {
					 int charsWritten = write(forward<T0>(arg0));
					 return charsWritten + write(forward<T1>(arg1), forward<Ts>(args)...);
				 }
};

class OutputFile : public OutputDevice {
	FILE* file;
	bool owner;

	void writeToDevice(uint count) override {
		fwrite(buffer, 1, count, file);
		fflush(file);
	}

	public:
	OutputFile(FILE* file = stdout, bool takeOwnership = false) : file(file), owner(takeOwnership) {}
	OutputFile(const char* fileName) : OutputFile(fopen(fileName, "w"), true) {}
	~OutputFile() override { flush(); if (owner) fclose(file); }
};

class OutputString : public OutputDevice {
	string& str;

	void writeToDevice(uint count) override { str.append(buffer, count); }

	public:
	OutputString(string& str) : OutputDevice(), str(str) {}
	~OutputString() override { flush(); }
};

unique_ptr<InputDevice> input;
unique_ptr<OutputDevice> output;

template <class... Ts> inline bool read(Ts&&... args) { return input->read(forward<Ts>(args)...); }
template <class... Ts> inline int write(Ts&&... args) { return output->write(forward<Ts>(args)...); }
template <class... Ts> inline int writeln(Ts&&... args) { return write(forward<Ts>(args)..., '\n'); }
void flush() { output->flush(); }

int cnt[200005];
int main() {
	//cin.tie(0)->sync_with_stdio(0);
	//cin.exceptions(cin.failbit);

	int n; cin >> n;
	ll ans = 0;

	rep(i, 1, n + 1) {
		int t; cin >> t;
		if (i - t >= 0) ans += cnt[i - t];
		if (t + i <= n) cnt[t + i]++;
	}
	cout << ans << '\n';
}


Measured memory_bytes: 4412