Problem p03231 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cfenv>
#include <chrono>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdint>
#include <cstdio>
#include <cstring>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits.h>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <time.h>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
// #include <bits/stdc++.h>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
long long MOD = 1000000000 + 7;
constexpr long long INF = numeric_limits<LL>::max();
const double PI = acos(-1);
#define fir first
#define sec second
#define thi third
#define debug(x) cerr << #x << ": " << x << '\n'
typedef pair<LL, LL> Pll;
typedef pair<LL, pair<LL, LL>> Ppll;
typedef pair<LL, pair<LL, bitset<100001>>> Pbll;
typedef pair<LL, pair<LL, vector<LL>>> Pvll;
typedef pair<LL, LL> Vec2;
struct Tll {
  LL first, second, third;
};
struct Fll {
  LL first, second, third, fourth;
};
typedef pair<LL, Tll> Ptll;
#define rep(i, rept) for (LL i = 0; i < rept; i++)
#define Mfor(i, mf) for (LL i = mf - 1; i >= 0; i--)
LL h, w, n, m, k, t, s, p, q, last, first, cnt, sum, ans, dp[10000], a[300000],
    b[300000];
string str[2], ss;
bool f[200];
char c, ch[100][100];
int di[4][2] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
struct Edge {
  LL to, cost;
};
vector<Edge> vec[200000];
vector<LL> v;
map<Pll, LL> ma;
multiset<LL> st;
void YN(bool f) {
  if (f)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
void yn(bool f) {
  if (f)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
LL gcd(LL a, LL b) {
  if (a < b)
    gcd(b, a);
  LL r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}
LL lcm(LL a, LL b) { return a * b / gcd(a, b); }

int main() {
  cin >> n >> m;
  cin >> str[0];
  cin >> str[1];
  h = gcd(n, m);
  rep(i, n / h - 1) {
    if (str[0][n / h * i] != str[1][m / h * i]) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << lcm(n, m) << endl;
  return 0;
}
// 5/15 3/9


Fixed submission:
#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <cctype>
#include <cfenv>
#include <chrono>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdint>
#include <cstdio>
#include <cstring>
#include <deque>
#include <fstream>
#include <functional>
#include <iomanip>
#include <iostream>
#include <limits.h>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <stdio.h>
#include <stdlib.h>
#include <string>
#include <time.h>
#include <tuple>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
// #include <bits/stdc++.h>
using namespace std;
using LL = long long;
using ULL = unsigned long long;
long long MOD = 1000000000 + 7;
constexpr long long INF = numeric_limits<LL>::max();
const double PI = acos(-1);
#define fir first
#define sec second
#define thi third
#define debug(x) cerr << #x << ": " << x << '\n'
typedef pair<LL, LL> Pll;
typedef pair<LL, pair<LL, LL>> Ppll;
typedef pair<LL, pair<LL, bitset<100001>>> Pbll;
typedef pair<LL, pair<LL, vector<LL>>> Pvll;
typedef pair<LL, LL> Vec2;
struct Tll {
  LL first, second, third;
};
struct Fll {
  LL first, second, third, fourth;
};
typedef pair<LL, Tll> Ptll;
#define rep(i, rept) for (LL i = 0; i < rept; i++)
#define Mfor(i, mf) for (LL i = mf - 1; i >= 0; i--)
LL h, w, n, m, k, t, s, p, q, last, first, cnt, sum, ans, dp[10000], a[300000],
    b[300000];
string str[2], ss;
bool f[200];
char c, ch[100][100];
int di[4][2] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
struct Edge {
  LL to, cost;
};
vector<Edge> vec[200000];
vector<LL> v;
map<Pll, LL> ma;
multiset<LL> st;
void YN(bool f) {
  if (f)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
void yn(bool f) {
  if (f)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
LL gcd(LL a, LL b) {
  if (a < b)
    gcd(b, a);
  LL r;
  while ((r = a % b)) {
    a = b;
    b = r;
  }
  return b;
}
LL lcm(LL a, LL b) { return a * b / gcd(a, b); }

int main() {
  cin >> n >> m;
  cin >> str[0];
  cin >> str[1];
  h = gcd(n, m);
  rep(i, h) {
    if (str[0][n / h * i] != str[1][m / h * i]) {
      cout << -1 << endl;
      return 0;
    }
  }
  cout << lcm(n, m) << endl;
  return 0;
}
// 5/15 3/9