#include <bits/stdc++.h>
using namespace std;
inline int read();
inline void write(int x);
const int M = 1024;
int save[M];
int main(void) {
  int n = read(), ans = 0;
  for (int i = 1; i <= n; i++) save[i] = read();
  int vecs = save[n] + 1;
  int r = n, l = 1, ready = 0;
  for (int vec = 1; vec < vecs; vec++) {
    if (save[l] <= ready)
      l++, r--;
    else if (ready + vecs - vec == save[r])
      ready++, ans += vecs - vec;
  }
  printf("%d\n", ans);
  r = n, l = 1, ready = 0;
  for (int vec = 1; vec < vecs; vec++) {
    if (save[l] <= ready)
      l++, r--;
    else if (ready + vecs - vec == save[r]) {
      for (int j = vec + 1; j <= vecs; j++) printf("%d %d\n", vec, j);
      ready++;
      ans += vecs - vec;
    }
  }
  return 0;
}
inline int read() {
  int x = 0, f = 1;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
inline void write(int x) {
  if (x < 0) putchar('-'), x = -x;
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
