stepik mail.ru c/cpp course

task1.cpp

#include <iostream>
#include <cstdlib>
#include <math.h>

using namespace std;

int main()
{
    char line[256]; 
    cin >> line;
    for (int i = 0; i < 256; i++)
    {
        if (line[i] == 0)
            break; 

        if (line[i] != '+' && (line[i] < '0' || line[i] > '9'))
        {
            cout << 0 << endl;
            return 0;
        }
    }
    long long int tn = atoll(line);
    if (tn <= 0)
    {
        cout << 0 << endl;
        return 0;
    }
    double index = (1 - sqrt(1 + 8.0 * tn)) / -2.0;
    if (index != (long long int)index)
    {
        cout << 0 << endl;
        return 0;
    }
    cout << (long long int)index << endl;
}