#include <bits/stdc++.h>
using namespace std;
const double pi = acos(0.0) * 2.0;
const long double eps = 1e-10;
const int step[8][2] = {{-1, 0}, {0, 1}, {1, 0},  {0, -1},
                        {-1, 1}, {1, 1}, {1, -1}, {-1, -1}};
template <class T>
inline T abs1(T a) {
  return a < 0 ? -a : a;
}
template <typename t, typename t1>
t min1(t a, t1 b) {
  return a < b ? a : b;
}
template <typename t, typename... arg>
t min1(t a, arg... arr) {
  return min1(a, min1(arr...));
}
template <typename t, typename t1>
t max1(t a, t1 b) {
  return a > b ? a : b;
}
template <typename t, typename... arg>
t max1(t a, arg... arr) {
  return max1(a, max1(arr...));
}
inline int jud(double a, double b) {
  if (abs(a) < eps && abs(b) < eps)
    return 0;
  else if (abs1(a - b) / max(abs1(a), abs1(b)) < 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) {
  if (na == 0) return 0;
  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;
}
template <class T>
inline T lowb(T num) {
  return num & (-num);
}
inline int bitnum(unsigned int nValue) { return __builtin_popcount(nValue); }
inline int bitnum(int nValue) { return __builtin_popcount(nValue); }
inline int bitnum(unsigned long long nValue) {
  return __builtin_popcount(nValue) + __builtin_popcount(nValue >> 32);
}
inline int bitnum(long long nValue) {
  return __builtin_popcount(nValue) + __builtin_popcount(nValue >> 32);
}
inline int bitmaxl(unsigned int a) {
  if (a == 0) return 0;
  return 32 - __builtin_clz(a);
}
inline int bitmaxl(int a) {
  if (a == 0) return 0;
  return 32 - __builtin_clz(a);
}
inline int bitmaxl(unsigned long long a) {
  int temp = a >> 32;
  if (temp) return 32 - __builtin_clz(temp) + 32;
  return bitmaxl(int(a));
}
inline int bitmaxl(long long a) {
  int temp = a >> 32;
  if (temp) return 32 - __builtin_clz(temp) + 32;
  return bitmaxl(int(a));
}
long long pow(long long n, long long m, long long mod = 0) {
  if (m < 0) return 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;
}
template <class t1, class t2>
inline void add(t1 &a, t2 b, int mod = -1) {
  if (mod == -1) mod = 1000000007;
  a += b;
  while (a >= mod) a -= mod;
  while (a < 0) a += mod;
}
template <class t>
void output1(t arr) {
  for (int i = 0; i < (int)arr.size(); i++) cerr << arr[i] << ' ';
  cerr << endl;
}
template <class t>
void output2(t arr) {
  for (int i = 0; i < (int)arr.size(); i++) output1(arr[i]);
}
const int maxn = 10;
int cnt[maxn][maxn];
int n, ln;
char a[maxn], b[maxn];
int have[maxn];
struct sap {
  struct edge {
    int to, cap, next;
  } e[300 * 2];
  int p, head[200], h[200], vh[200];
  int n, m, s, t, maxFlow;
  inline int min(int a, int b) { return a < b ? a : b; }
  inline void addedge(int a, int b, int c) {
    e[p].to = b, e[p].cap = c, e[p].next = head[a], head[a] = p++;
    e[p].to = a, e[p].cap = 0, e[p].next = head[b], head[b] = p++;
  }
  int dfs(int u, int pre) {
    if (u == t) return pre;
    int now, aug, mh = n + 10, to, tmp = pre;
    for (now = head[u]; now; now = e[now].next) {
      to = e[now].to;
      if (e[now].cap) {
        if (pre && h[u] == h[to] + 1) {
          aug = dfs(to, min(pre, e[now].cap));
          pre -= aug;
          e[now].cap -= aug;
          e[now ^ 1].cap += aug;
          if (h[s] >= n) return tmp - pre;
        }
        mh = min(mh, h[to]);
      }
    }
    if (tmp - pre > 0) return tmp - pre;
    --vh[h[u]];
    if (!vh[h[u]]) {
      h[s] = n;
      return 0;
    }
    ++vh[h[u] = mh + 1];
    return 0;
  }
  void init() {
    maxFlow = 0;
    memset(h, 0, sizeof(h));
    memset(vh, 0, sizeof(vh));
    vh[0] = n;
  }
  void SAP() {
    init();
    while (h[s] < n) maxFlow += dfs(s, 0x5fffffff);
  }
  void pre(int nn, int ss = -1, int tt = -1) {
    p = 2;
    memset(head, 0, sizeof(head));
    n = nn;
    if (ss == -1)
      s = 0;
    else
      s = ss;
    if (tt == -1)
      t = n - 1;
    else
      t = tt;
  }
} g;
int fa[maxn];
bool dfs(int deep) {
  if (deep == ln + 1) {
    int can[maxn];
    memset(can, 0, sizeof(can));
    can[1] = 1;
    for (int j = 0; j < ln; j++)
      for (int i = 1; i <= ln; i++)
        if (can[fa[i]]) can[i] = 1;
    for (int i = 1; i <= ln; i++)
      if (!can[i]) return 0;
    int src = 42, sink = 43;
    g.pre(44, src, sink);
    for (int j = 1; j <= 6; j++) g.addedge(j + 35, sink, have[j] - 1);
    for (int i = 1; i <= 6; i++)
      for (int j = 1; j <= 6; j++)
        if (i <= j) {
          g.addedge((i - 1) * 6 + j - 1, 35 + i, 0x5fffffff);
          g.addedge((i - 1) * 6 + j - 1, 35 + j, 0x5fffffff);
          g.addedge(src, (i - 1) * 6 + j - 1, cnt[i][j]);
        }
    g.SAP();
    if (g.maxFlow == n - ln) return 1;
    return 0;
  }
  for (int i = 1; i <= ln; i++)
    if (i != deep && cnt[deep][i]) {
      cnt[deep][i]--;
      cnt[i][deep]--;
      fa[deep] = i;
      bool flag = dfs(deep + 1);
      cnt[deep][i]++;
      cnt[i][deep]++;
      if (flag) return 1;
    }
  return 0;
}
int main() {
  ios_base::sync_with_stdio(0);
  scanf("%d", &n);
  for (int i = n; i; i /= 10, ln++)
    ;
  for (int j = 1; j <= n; j++) {
    int rcnt = 0;
    for (int i = j; i; i /= 10, rcnt++)
      ;
    have[rcnt]++;
  }
  bool judg = 0;
  for (int i = 0; i < n - 1; i++) {
    scanf("%s%s", a, b);
    int la = strlen(a), lb = strlen(b);
    if (n == 1000 && (string)a == "???" && (string)b == "???" && i == 0)
      judg = 1;
    cnt[la][lb]++;
    if (la != lb) cnt[lb][la]++;
  }
  fa[1] = 1;
  if (!dfs(2)) {
    cout << -1 << endl;
    return 0;
  }
  int root[maxn];
  for (int i = 1; i <= ln; i++) {
    root[i] = 1;
    for (int j = 1; j < i; j++) root[i] *= 10;
  }
  for (int i = 2; i <= ln; i++) {
    printf("%d %d\n", root[i], root[fa[i]]);
  }
  for (int i = 36; i < 42; i++) {
    int no = i - 35;
    int now = root[no] + 1;
    for (int j = g.head[i]; j; j = g.e[j].next)
      if (g.e[j].to < i) {
        int no1 = g.e[j].to % 6 + 1, no2 = g.e[j].to / 6 + 1;
        if (no1 == no) swap(no1, no2);
        for (int k = 0; k < g.e[j].cap; k++) {
          printf("%d %d\n", now, root[no1]);
          now++;
        }
      }
  }
  return 0;
}
