Problem p03231 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define P pair<ll, ll>
#define FOR(I, A, B) for (ll I = ll(A); I < ll(B); ++I)
#define FORR(I, A, B) for (ll I = ll((B)-1); I >= ll(A); --I)
#define TO(x, t, f) ((x) ? (t) : (f))
#define SORT(x) (sort(x.begin(), x.end())) // 0 2 2 3 4 5 8 9
#define POSL(x, v)                                                             \
  (lower_bound(x.begin(), x.end(), v) - x.begin()) // xi>=v  x is sorted
#define POSU(x, v)                                                             \
  (upper_bound(x.begin(), x.end(), v) - x.begin()) // xi>v  x is sorted
#define NUM(x, v) (POSU(x, v) - POSL(x, v))        // x is sorted
#define REV(x) (reverse(x.begin(), x.end()))       // reverse
ll gcd_(ll a, ll b) {
  if (a % b == 0)
    return b;
  return gcd_(b, a % b);
}
ll lcm_(ll a, ll b) {
  ll c = gcd_(a, b);
  return ((a / c) * (b / c) * c);
}
#define NEXTP(x) next_permutation(x.begin(), x.end())
const ll INF = ll(1e16) + ll(7);
const ll MOD = 1000000007LL;
#define out(a) cout << fixed << setprecision((a))
// tie(a,b,c) = make_tuple(10,9,87);
#define pop_(a) __builtin_popcount((a))
ll keta(ll a) {
  ll r = 0;
  while (a) {
    a /= 10;
    r++;
  }
  return r;
}

int main() {
  ll n, m;
  string s, t;

  cin >> n >> m >> s >> t;

  ll len = lcm_(n, m);

  FOR(i, 0, 100000) {
    if (len > n * m)
      break;
    bool ok = true;
    FOR(k, 0, n) {
      if (k * m % n == 0) {
        if (s[k] != t[k * m / n])
          ok = false;
      }
    }
    if (ok) {
      cout << len << endl;
      return 0;
    }
    len += lcm_(n, m);
  }
  cout << -1 << endl;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define P pair<ll, ll>
#define FOR(I, A, B) for (ll I = ll(A); I < ll(B); ++I)
#define FORR(I, A, B) for (ll I = ll((B)-1); I >= ll(A); --I)
#define TO(x, t, f) ((x) ? (t) : (f))
#define SORT(x) (sort(x.begin(), x.end())) // 0 2 2 3 4 5 8 9
#define POSL(x, v)                                                             \
  (lower_bound(x.begin(), x.end(), v) - x.begin()) // xi>=v  x is sorted
#define POSU(x, v)                                                             \
  (upper_bound(x.begin(), x.end(), v) - x.begin()) // xi>v  x is sorted
#define NUM(x, v) (POSU(x, v) - POSL(x, v))        // x is sorted
#define REV(x) (reverse(x.begin(), x.end()))       // reverse
ll gcd_(ll a, ll b) {
  if (a % b == 0)
    return b;
  return gcd_(b, a % b);
}
ll lcm_(ll a, ll b) {
  ll c = gcd_(a, b);
  return ((a / c) * (b / c) * c);
}
#define NEXTP(x) next_permutation(x.begin(), x.end())
const ll INF = ll(1e16) + ll(7);
const ll MOD = 1000000007LL;
#define out(a) cout << fixed << setprecision((a))
// tie(a,b,c) = make_tuple(10,9,87);
#define pop_(a) __builtin_popcount((a))
ll keta(ll a) {
  ll r = 0;
  while (a) {
    a /= 10;
    r++;
  }
  return r;
}

int main() {
  ll n, m;
  string s, t;

  cin >> n >> m >> s >> t;

  ll len = lcm_(n, m);

  FOR(i, 0, 100000) {
    if (len > n * m)
      break;
    bool ok = true;
    FOR(k, 0, n) {
      if (k * m % n == 0) {
        if (s[k] != t[k * m / n])
          ok = false;
        if (not ok)
          break;
      }
    }
    if (ok) {
      cout << len << endl;
      return 0;
    }
    len += lcm_(n, m);
  }
  cout << -1 << endl;
}