Alien Invasion II

input/code.cpp: In function 'int main()':
input/code.cpp:11:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < str.size(); i++) {
                     ~~^~~~~~~~~~~~

#include <iostream>
#include <string>

using namespace std;

int main() {
    int x;
    cin >> x;
    string str = to_string(x);
    int s = 0;
    for (int i = 0; i < str.size(); i++) {
        s += (int) i;
    }
    int add = 3 - (s % 3);
    str = to_string(add) + str;
    cout << str << endl;
    x = stoi(str);
    cout << 3 << " " << x / 3;
    return 0;
}

Test 1

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  stoi

Test 2

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  stoi

Test 3

Verdict: RUNTIME ERROR

terminate called after throwing an instance of 'std::out_of_range'
  what():  stoi