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public static void main(String[] args) throws IOException {

        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        StreamTokenizer in = new StreamTokenizer(br);

        PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));

        in.nextToken();
        int n = (int) in.nval;

        // 创建HashMap：键=差值diff（Integer），值=该diff出现的次数（Integer）
        Map<Integer, Integer> hMap = new HashMap<>();

        for (int i = 0; i < n; i++) {
            in.nextToken();
            int a = (int) in.nval;

            // diff = 前整数 - 它的位置索引
            int diff = a - i;
            hMap.put(diff, hMap.getOrDefault(diff, 0) + 1);
        }
        long total = 0;

        // 遍历Map中所有diff的出现次数
        for (int count : hMap.values()) {
            // 只有出现次数>1的diff，才能组成数对（至少2个元素才能配对）
            if (count > 1) {
                // 相同 diff 的数，两两之间都满足题目要求的条件，
                // 所以统计每个 diff 的出现次数，再算组合数就是答案。
                total += (long) count * (count - 1) / 2;
            }
        }

        out.println(total);

        out.flush();
        out.close();
        br.close();
    }