Problem p03231 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define PI 3.14159265358979323846
#define MAXINF (1e18L)
#define INF (1e9L)
#define EPS (1e-9)
#define MOD ((ll)(1e9 + 7))
#define REP(i, n) for (int i = 0; i < int(n); ++i)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define RREP(i, n) for (int i = int(n) - 1; i >= 0; --i)
#define ALL(v) v.begin(), v.end()
#define FIND(v, x) (binary_search(ALL(v), (x)))
#define SORT(v) sort(ALL(v))
#define RSORT(v)                                                               \
  sort(ALL(v));                                                                \
  reverse(ALL(v))
#define DEBUG(x) cerr << #x << ": " << x << endl;
#define DEBUG_VEC(v)                                                           \
  cerr << #v << ":";                                                           \
  for (int i = 0; i < v.size(); i++)                                           \
    cerr << " " << v[i];                                                       \
  cerr << endl
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl
#define YES(n) cout << ((n) ? "YES" : "NO") << endl
#define pb push_back
#define fi first
#define se second
using namespace std;
template <class A> void pr(A a) { cout << (a) << endl; }
template <class A, class B> void pr(A a, B b) {
  cout << a << " ";
  pr(b);
}
template <class A, class B, class C> void pr(A a, B b, C c) {
  cout << a << " ";
  pr(b, c);
}
template <class A, class B, class C, class D> void pr(A a, B b, C c, D d) {
  cout << a << " ";
  pr(b, c, d);
}
template <class T> inline bool chmin(T &a, T b) {
  return a > b ? a = b, true : false;
}
template <class T> inline bool chmax(T &a, T b) {
  return a < b ? a = b, true : false;
}
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) { return a * (b / gcd(a, b)); }

int main(void) {
  ll N, M;
  cin >> N >> M;
  string S, T;
  cin >> S >> T;

  ll l = lcm(N, M);
  ll k = gcd(N, M);

  while (l <= N * M) {
    bool flag = true;
    for (size_t i = 0; i < min(N, M); i++) {
      if (S[i * (N / k)] != T[i * (M / k)]) {
        flag = false;
        break;
      }
    }

    if (flag) {
      pr(l);
      return 0;
    }
    l += lcm(N, M);
  }

  pr(-1);
}

Fixed submission:
#include <bits/stdc++.h>
#define PI 3.14159265358979323846
#define MAXINF (1e18L)
#define INF (1e9L)
#define EPS (1e-9)
#define MOD ((ll)(1e9 + 7))
#define REP(i, n) for (int i = 0; i < int(n); ++i)
#define Rep(i, sta, n) for (int i = sta; i < n; i++)
#define RREP(i, n) for (int i = int(n) - 1; i >= 0; --i)
#define ALL(v) v.begin(), v.end()
#define FIND(v, x) (binary_search(ALL(v), (x)))
#define SORT(v) sort(ALL(v))
#define RSORT(v)                                                               \
  sort(ALL(v));                                                                \
  reverse(ALL(v))
#define DEBUG(x) cerr << #x << ": " << x << endl;
#define DEBUG_VEC(v)                                                           \
  cerr << #v << ":";                                                           \
  for (int i = 0; i < v.size(); i++)                                           \
    cerr << " " << v[i];                                                       \
  cerr << endl
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl
#define YES(n) cout << ((n) ? "YES" : "NO") << endl
#define pb push_back
#define fi first
#define se second
using namespace std;
template <class A> void pr(A a) { cout << (a) << endl; }
template <class A, class B> void pr(A a, B b) {
  cout << a << " ";
  pr(b);
}
template <class A, class B, class C> void pr(A a, B b, C c) {
  cout << a << " ";
  pr(b, c);
}
template <class A, class B, class C, class D> void pr(A a, B b, C c, D d) {
  cout << a << " ";
  pr(b, c, d);
}
template <class T> inline bool chmin(T &a, T b) {
  return a > b ? a = b, true : false;
}
template <class T> inline bool chmax(T &a, T b) {
  return a < b ? a = b, true : false;
}
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) { return a * (b / gcd(a, b)); }

int main(void) {
  ll N, M;
  cin >> N >> M;
  string S, T;
  cin >> S >> T;

  ll l = lcm(N, M);
  ll k = gcd(N, M);

  while (l <= N * M) {
    bool flag = true;
    for (size_t i = 0; i < min(N, M); i++) {
      if (i * (N / k) >= N || i * (M / k) >= M)
        break;
      if (S[i * (N / k)] != T[i * (M / k)]) {
        flag = false;
        break;
      }
    }

    if (flag) {
      pr(l);
      return 0;
    }
    l += lcm(N, M);
  }

  pr(-1);
}