#include <bits/stdc++.h>
using namespace std;
const double pi = acos(0.0) * 2.0;
const double eps = 1e-12;
const int step[8][2] = {{1, 0}, {-1, 0}, {0, 1},  {0, -1},
                        {1, 1}, {1, -1}, {-1, 1}, {-1, -1}};
template <class T>
inline T abs1(T a) {
  return a < 0 ? -a : a;
}
template <class T>
inline T max1(T a, T b, T c = -1234567, T d = -1234567) {
  T ans = a > b ? a : b;
  if (c != -1234567) ans = max1(ans, c);
  if (d != -1234567) ans = max1(ans, d);
  return ans;
}
template <class T>
inline T min1(T a, T b, T c = -1234567, T d = -1234567) {
  T ans = a < b ? a : b;
  if (c != -1234567) ans = min(ans, c);
  if (d != -1234567) ans = min(ans, d);
  return ans;
}
template <class T>
inline T gcd1(T a, T b) {
  if (a < b) swap(a, b);
  if (a % b == 0) return b;
  return gcd1(b, a % b);
}
template <class T>
inline T lb(T num) {
  return num & (-num);
}
inline int jud(double a, double b) {
  if (abs(a) < eps && abs(b) < eps)
    return 0;
  else if (abs1(a - b) / abs1(a) < eps)
    return 0;
  if (a < b) return -1;
  return 1;
}
template <typename t>
inline int jud(t a, t b) {
  if (a < b) return -1;
  if (a == b) return 0;
  return 1;
}
template <typename it, typename t1>
inline int find(t1 val, it a, int na, bool f_small = 1, bool f_lb = 1) {
  int be = 0, en = na - 1;
  if (*a <= *(a + na - 1)) {
    if (f_lb == 0)
      while (be < en) {
        int mid = (be + en + 1) / 2;
        if (jud(*(a + mid), val) != 1)
          be = mid;
        else
          en = mid - 1;
      }
    else
      while (be < en) {
        int mid = (be + en) / 2;
        if (jud(*(a + mid), val) != -1)
          en = mid;
        else
          be = mid + 1;
      }
    if (f_small && jud(*(a + be), val) == 1) be--;
    if (!f_small && jud(*(a + be), val) == -1) be++;
  } else {
    if (f_lb)
      while (be < en) {
        int mid = (be + en + 1) / 2;
        if (jud(*(a + mid), val) != -1)
          be = mid;
        else
          en = mid - 1;
      }
    else
      while (be < en) {
        int mid = (be + en) / 2;
        if (jud(*(a + mid), val) != 1)
          en = mid;
        else
          be = mid + 1;
      }
    if (!f_small && jud(*(a + be), val) == -1) be--;
    if (f_small && jud(*(a + be), val) == 1) be++;
  }
  return be;
}
inline int bitnum(unsigned long long nValue) {
  nValue = ((0xaaaaaaaaaaaaaaaaull & nValue) >> 1) +
           (0x5555555555555555ull & nValue);
  nValue = ((0xccccccccccccccccull & nValue) >> 2) +
           (0x3333333333333333ull & nValue);
  nValue = ((0xf0f0f0f0f0f0f0f0ull & nValue) >> 4) +
           (0x0f0f0f0f0f0f0f0full & nValue);
  nValue = ((0xff00ff00ff00ff00ull & nValue) >> 8) +
           (0x00ff00ff00ff00ffull & nValue);
  nValue = ((0xffff0000ffff0000ull & nValue) >> 16) +
           (0x0000ffff0000ffffull & nValue);
  nValue = ((0xffffffff00000000ull & nValue) >> 32) +
           (0x00000000ffffffffull & nValue);
  return nValue;
}
long long pow(long long n, long long m, long long mod = 0) {
  long long ans = 1;
  long long k = n;
  while (m) {
    if (m & 1) {
      ans *= k;
      if (mod) ans %= mod;
    }
    k *= k;
    if (mod) k %= mod;
    m >>= 1;
  }
  return ans;
}
const int maxn = 1100;
int orig[maxn], n;
int cnt[maxn];
int ans[maxn * maxn][2], lans = 0;
bool f[maxn];
inline void findnxt(int &no) {
  while (f[no] == 0) no++;
  f[no] = 0;
}
inline void relax(int &a, int &b, int &c) {
  if (orig[b] < orig[a]) swap(a, b);
  if (orig[c] < orig[b]) swap(b, c);
  if (orig[b] < orig[a]) swap(a, b);
}
int main() {
  ios_base::sync_with_stdio(0);
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", orig + i);
  int cnt = 0;
  for (int i = 0; i < n; ++i) {
    if (orig[i])
      f[i] = 1, cnt++;
    else
      f[i] = 0;
  }
  if (cnt < 2) {
    cout << -1 << endl;
    return 0;
  }
  f[n] = f[n + 1] = f[n + 2] = 1;
  orig[n] = orig[n + 1] = orig[n + 2] = 100000007;
  f[1] = f[2] = 0;
  int a = 0, b = 1, c = 2;
  for (; c < n;) {
    findnxt(a);
    relax(a, b, c);
    if (c >= n) break;
    while (orig[a]) {
      int temp = orig[b] / orig[a];
      for (; temp; temp >>= 1) {
        if (temp & 1) {
          ans[lans][1] = b;
          orig[b] -= orig[a];
        } else {
          ans[lans][1] = c;
          orig[c] -= orig[a];
        }
        ans[lans++][0] = a;
        orig[a] <<= 1;
      }
      relax(a, b, c);
    }
  }
  printf("%d\n", lans);
  for (int i = 0; i < lans; i++)
    printf("%d %d\n", ans[i][0] + 1, ans[i][1] + 1);
  return 0;
}
