String padding

#include <iostream>
#include <string>

using namespace std;

string chessPawnTour(long long  n, long long m) {
    if ((n * m) % 2 != 0) {
        return "-1";
    }

    string moves;

    for (int i = 0; i < n; ++i) {
        if (i % 2 == 0) {
            moves.append(m - 1, 'R');
            if (i < n - 1) {
                moves += 'D';
            }
        } else {
            moves.append(m - 1, 'L');
            if (i < n - 1) {
                moves += 'D';
            }
        }
    }

    moves.append(n - 1, 'U');
    moves += 'L';

    return moves;
}

int main() {
    long long n, m;
    cin >> n >> m;
    string result = chessPawnTour(n, m);
    cout << result << endl;
    return 0;
}

Test 1

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 2

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 3

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 4

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 5

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 6

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 7

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 8

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 9

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 10

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 11

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 12

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 13

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 14

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 15

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 16

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 17

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux

Test 18

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::length_error'
  what():  basic_string::_M_replace_aux