Problem p03231 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, s, N) for (ll i{s}; i < N; i++)
#define rem(i, N, s) for (ll i{N}; i > s; i--)
using namespace std;
using ll = long long int;
using ld = long double;
using P = pair<ll, ll>;
const int MOD = (int)1e9 + 7;
const string rt = "\n";
const string sp = " "; /* 最大公約数 */
template <typename T> T gcd(T a, T b) {
  return b != 0 ? gcd(b, a % b) : a;
} /* UnionFind */
template <typename T> struct UnionFind {
  vector<T> par;
  UnionFind(T n) : par(n, -1) {}
  void init(T n) { par.assign(n, -1); }
  T root(T x) {
    if (par[x] < 0)
      return x;
    else
      return par[x] = root(par[x]);
  }
  bool issame(T x, T y) { return root(x) == root(y); }
  bool merge(T x, T y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (par[x] > par[y])
      swap(x, y);
    par[x] += par[y];
    par[y] = x;
    return true;
  }
  int size(int x) { return -par[root(x)]; }
}; /* コンビネーション */
template <typename T> ll combpm(T N_, T C_) {
  const int NUM_ = 400001;
  static ll fact[NUM_ + 1], factr[NUM_ + 1], inv[NUM_ + 1];
  if (fact[0] == 0) {
    inv[1] = fact[0] = factr[0] = 1;
    for (int i = 2; i <= NUM_; ++i)
      inv[i] = inv[MOD % i] * (MOD - MOD / i) % MOD;
    for (int i = 1; i <= NUM_; ++i)
      fact[i] = fact[i - 1] * i % MOD, factr[i] = factr[i - 1] * inv[i] % MOD;
  }
  if (C_ < 0 || C_ > N_)
    return 0;
  return factr[C_] * fact[N_] % MOD * factr[N_ - C_] % MOD;
} /* 多次元ベクター */
template <class T> vector<T> mvec(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto mvec(size_t a, Ts... ts) {
  return vector<decltype(mvec<T>(ts...))>(a, mvec<T>(ts...));
} /* テスト関数 */
void test(ll n) { cout << "test" << n << endl; } /* 小数点以下 */
void fixsp(ll n) { cout << fixed << setprecision(n); }
void defsp(ll n) {
  cout << defaultfloat << setprecision(n);
} /* 重み付きUnionFind */
struct WUnionFind {
  vector<int> par;
  vector<int> rank;
  WUnionFind(int n = 1) { init(n); }
  void init(int n = 1) {
    par.resize(n);
    rank.resize(n);
    for (int i = 0; i < n; ++i)
      par[i] = i, rank[i] = 0;
  }
  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }
  bool issame(int x, int y) { return root(x) == root(y); }
  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    return true;
  }
}; /* IO高速化 */
void fastio() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
}
/* ここからコード開始 */

int main() {
  fastio;
  ll N, M;
  cin >> N >> M;
  string S, T;
  cin >> S >> T;
  auto X = mvec<char>(N * M);
  rep(i, 0, N) { X[i * M] = S[i]; }
  rep(i, 0, M) {
    if (X[i * N] == T[i] || X[i * N] == 0) {
      X[i * N] == T[i];
    } else {
      cout << -1 << rt;
      return 0;
    }
  }
  cout << N * M / gcd(N, M) << rt;
}

Fixed submission:
#include <bits/stdc++.h>
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define rep(i, s, N) for (ll i{s}; i < N; i++)
#define rem(i, N, s) for (ll i{N}; i > s; i--)
using namespace std;
using ll = long long int;
using ld = long double;
using P = pair<ll, ll>;
const int MOD = (int)1e9 + 7;
const string rt = "\n";
const string sp = " "; /* 最大公約数 */
template <typename T> T gcd(T a, T b) {
  return b != 0 ? gcd(b, a % b) : a;
} /* UnionFind */
template <typename T> struct UnionFind {
  vector<T> par;
  UnionFind(T n) : par(n, -1) {}
  void init(T n) { par.assign(n, -1); }
  T root(T x) {
    if (par[x] < 0)
      return x;
    else
      return par[x] = root(par[x]);
  }
  bool issame(T x, T y) { return root(x) == root(y); }
  bool merge(T x, T y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (par[x] > par[y])
      swap(x, y);
    par[x] += par[y];
    par[y] = x;
    return true;
  }
  int size(int x) { return -par[root(x)]; }
}; /* コンビネーション */
template <typename T> ll combpm(T N_, T C_) {
  const int NUM_ = 400001;
  static ll fact[NUM_ + 1], factr[NUM_ + 1], inv[NUM_ + 1];
  if (fact[0] == 0) {
    inv[1] = fact[0] = factr[0] = 1;
    for (int i = 2; i <= NUM_; ++i)
      inv[i] = inv[MOD % i] * (MOD - MOD / i) % MOD;
    for (int i = 1; i <= NUM_; ++i)
      fact[i] = fact[i - 1] * i % MOD, factr[i] = factr[i - 1] * inv[i] % MOD;
  }
  if (C_ < 0 || C_ > N_)
    return 0;
  return factr[C_] * fact[N_] % MOD * factr[N_ - C_] % MOD;
} /* 多次元ベクター */
template <class T> vector<T> mvec(size_t a) { return vector<T>(a); }
template <class T, class... Ts> auto mvec(size_t a, Ts... ts) {
  return vector<decltype(mvec<T>(ts...))>(a, mvec<T>(ts...));
} /* テスト関数 */
void test(ll n) { cout << "test" << n << endl; } /* 小数点以下 */
void fixsp(ll n) { cout << fixed << setprecision(n); }
void defsp(ll n) {
  cout << defaultfloat << setprecision(n);
} /* 重み付きUnionFind */
struct WUnionFind {
  vector<int> par;
  vector<int> rank;
  WUnionFind(int n = 1) { init(n); }
  void init(int n = 1) {
    par.resize(n);
    rank.resize(n);
    for (int i = 0; i < n; ++i)
      par[i] = i, rank[i] = 0;
  }
  int root(int x) {
    if (par[x] == x) {
      return x;
    } else {
      int r = root(par[x]);
      return par[x] = r;
    }
  }
  bool issame(int x, int y) { return root(x) == root(y); }
  bool merge(int x, int y) {
    x = root(x);
    y = root(y);
    if (x == y)
      return false;
    if (rank[x] < rank[y])
      swap(x, y);
    if (rank[x] == rank[y])
      ++rank[x];
    par[y] = x;
    return true;
  }
}; /* IO高速化 */
void fastio() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
}
/* ここからコード開始 */

int main() {
  fastio;
  ll N, M;
  cin >> N >> M;
  string S, T;
  cin >> S >> T;
  ll g = gcd(N, M);
  rep(i, 0, g) {
    if (S[i * N / g] == T[i * M / g]) {
    } else {
      cout << -1 << rt;
      return 0;
    }
  }
  cout << N * M / gcd(N, M) << rt;
}