#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using db = double;
using str = string;
using pi = pair<int, int>;
using pl = pair<ll, ll>;
using pd = pair<db, db>;
using vi = vector<int>;
using vb = vector<bool>;
using vl = vector<ll>;
using vd = vector<db>;
using vs = vector<str>;
using vpi = vector<pi>;
using vpl = vector<pl>;
using vpd = vector<pd>;
template <class T>
using V = vector<T>;
template <class T, size_t SZ>
using AR = array<T, SZ>;
const int MOD = 1e9 + 7;
const ll INF = 1e18;
const ll infl = 1e18 + 5;
const int infi = 1e9 + 5;
const int xd[4] = {1, 0, -1, 0}, yd[4] = {0, 1, 0, -1};
mt19937 rng((uint32_t)chrono::steady_clock::now().time_since_epoch().count());
constexpr int pct(int x) { return __builtin_popcount(x); }
constexpr int bits(int x) { return 31 - __builtin_clz(x); }
ll cdiv(ll a, ll b) { return a / b + ((a ^ b) > 0 && a % b); }
ll fdiv(ll a, ll b) { return a / b - ((a ^ b) < 0 && a % b); }
template <class T>
bool ckmin(T& a, const T& b) {
  return b < a ? a = b, 1 : 0;
}
template <class T>
bool ckmax(T& a, const T& b) {
  return a < b ? a = b, 1 : 0;
}
template <class T, class U>
T fstTrue(T lo, T hi, U f) {
  hi++;
  assert(lo <= hi);
  while (lo < hi) {
    T mid = lo + (hi - lo) / 2;
    f(mid) ? hi = mid : lo = mid + 1;
  }
  return lo;
}
template <class T, class U>
T lstTrue(T lo, T hi, U f) {
  lo--;
  assert(lo <= hi);
  while (lo < hi) {
    T mid = lo + (hi - lo + 1) / 2;
    f(mid) ? lo = mid : hi = mid - 1;
  }
  return lo;
}
template <class T>
void remDup(vector<T>& v) {
  sort(begin(v), end(v));
  v.erase(unique(begin(v), end(v)), end(v));
}
template <class T, class U>
void erase(T& t, const U& u) {
  auto it = t.find(u);
  assert(it != end(t));
  t.erase(u);
}
template <class T>
void re(T& x) {
  cin >> x;
}
void re(db& d) {
  str t;
  re(t);
  d = stod(t);
}
void re(ld& d) {
  str t;
  re(t);
  d = stold(t);
}
template <class T, class... U>
void re(T& t, U&... u) {
  re(t);
  re(u...);
}
template <class T>
void re(complex<T>& c) {
  T a, b;
  re(a, b);
  c = {a, b};
}
template <class T, class U>
void re(pair<T, U>& p) {
  re(p.f, p.s);
}
template <class T>
void re(vector<T>& x) {
  for (auto& a : x) re(a);
}
template <class T, size_t SZ>
void re(AR<T, SZ>& x) {
  for (auto& a : x) re(a);
}
str to_string(char c) { return str(1, c); }
str to_string(const char* s) { return (str)s; }
str to_string(str s) { return s; }
str to_string(bool b) { return to_string((int)b); }
template <class T>
str to_string(complex<T> c) {
  stringstream ss;
  ss << c;
  return ss.str();
}
str to_string(vector<bool> v) {
  str res = "{";
  for (int i = (0); i < ((int)(v).size()); ++i) res += char('0' + v[i]);
  res += "}";
  return res;
}
template <size_t SZ>
str to_string(bitset<SZ> b) {
  str res = "";
  for (int i = (0); i < (SZ); ++i) res += char('0' + b[i]);
  return res;
}
template <class T>
str to_string(T v) {
  bool fst = 1;
  str res = "";
  for (const auto& x : v) {
    if (!fst) res += " ";
    fst = 0;
    res += to_string(x);
  }
  return res;
}
template <class T, class U>
str to_string(pair<T, U> p) {
  return to_string(p.f) + " " + to_string(p.s);
}
template <class T>
void pr(T x) {
  cout << to_string(x);
}
template <class T, class... U>
void pr(const T& t, const U&... u) {
  pr(t);
  pr(u...);
}
void ps() { pr("\n"); }
template <class T, class... U>
void ps(const T& t, const U&... u) {
  pr(t);
  if (sizeof...(u)) pr(" ");
  ps(u...);
}
void pdbg(ll t) { cerr << t; }
void pdbg(int t) { cerr << t; }
void pdbg(str t) { cerr << t; }
void pdbg(char t) { cerr << t; }
void pdbg(ld t) { cerr << t; }
void pdbg(db t) { cerr << t; }
void pdbg(ull t) { cerr << t; }
template <class T, class V>
void pdbg(pair<T, V> p) {
  cerr << "{";
  pdbg(p.ff);
  cerr << ",";
  pdbg(p.ss);
  cerr << "}";
}
template <class T>
void pdbg(vector<T> v) {
  cerr << "[ ";
  for (T i : v) {
    pdbg(i);
    cerr << " ";
  }
  cerr << "]";
}
template <class T>
void pdbg(set<T> v) {
  cerr << "[ ";
  for (T i : v) {
    pdbg(i);
    cerr << " ";
  }
  cerr << "]";
}
template <class T>
void pdbg(multiset<T> v) {
  cerr << "[ ";
  for (T i : v) {
    pdbg(i);
    cerr << " ";
  }
  cerr << "]";
}
template <class T, class V>
void pdbg(map<T, V> v) {
  cerr << "[ ";
  for (auto i : v) {
    pdbg(i);
    cerr << " ";
  }
  cerr << "]";
}
void solve() {
  ll n;
  re(n);
  str s;
  re(s);
  str t;
  re(t);
  ll a1 = 0, b1 = 0;
  ll a2 = 0, b2 = 0;
  for (int i = (0); i < (n); ++i) {
    if (s[i] == 'a') {
      a1 += 1;
    } else {
      b1 += 1;
    }
    if (t[i] == 'a') {
      a2 += 1;
    } else {
      b2 += 1;
    }
  }
  if ((a1 + a2) % 2 == 0 && (b1 + b2) % 2 == 0) {
    ll ans = 0;
    ll ab = 0, ba = 0;
    vl aa1;
    vl aa2;
    vl bb1;
    vl bb2;
    for (int i = (0); i < (n); ++i) {
      if (s[i] == 'a' && t[i] == 'b') {
        ab += 1;
        aa1.push_back(i + 1);
        bb2.push_back(i + 1);
      } else if (s[i] == 'b' && t[i] == 'a') {
        ba += 1;
        bb1.push_back(i + 1);
        aa2.push_back(i + 1);
      }
    }
    ans += ab / 2;
    ans += ba / 2;
    if (ab % 2 == 1) {
      ans += 2;
    }
    ps(ans);
    if (aa1.size() != 0 || aa2.size() != 0) {
      while (aa1.size() > 1) {
        cout << aa1[aa1.size() - 1] << " " << bb2[aa1.size() - 2] << endl;
        aa1.pop_back();
        bb2.pop_back();
        aa1.pop_back();
        bb2.pop_back();
      }
      while (aa2.size() > 1) {
        cout << aa2[aa2.size() - 1] << " " << bb1[aa2.size() - 2] << endl;
        aa2.pop_back();
        bb1.pop_back();
        aa2.pop_back();
        bb1.pop_back();
      }
      if (aa1.size() == 1) {
        cout << aa1[0] << " " << bb2[0] << endl;
        cout << aa1[0] << " " << bb1[0] << endl;
        aa1.pop_back();
        bb1.pop_back();
        aa2.pop_back();
        bb2.pop_back();
      }
    }
  } else {
    cout << -1;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ll T = 1;
  for (int i = (0); i < (T); ++i) solve();
  return 0;
}
