#include <bits/stdc++.h>
using namespace std;
using ll = long long;
vector<pair<int, int>> ans;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  int n;
  cin >> n;
  vector<int> a(n);
  for (int i = 0; i < n; i++) cin >> a[i];
  reverse(a.begin(), a.end());
  vector<pair<int, int>> ans;
  int m = a[0] + 1;
  vector<int> cnt(m);
  int l = 0, r = n;
  int st = 0;
  int en = m;
  while (l < r) {
    if (cnt[en - 1] == a[r - 1]) {
      --en;
      --r;
      continue;
    }
    if (a[l] >= en) l++;
    for (int i = 0; i < a[l] - st; i++) {
      ans.emplace_back(st, st + i + 1);
      cnt[st]++;
      cnt[st + i + 1]++;
    }
    en = a[l] + 1;
    st++;
  }
  cout << ans.size() << endl;
  for (auto p : ans) cout << p.first + 1 << " " << p.second + 1 << endl;
}
