Here's my solution, can't figure out, what's wrong, it passes all the tests I can think of.
Help me, please.

#include <iostream>
#include <string>

int state;

long long base(int b, std::string str)
{
    int size = str.size();
    int i = 0;
    long long res = 0;
    while(i < size)
    {
        res *=  b;
        if(str[i] >= 'A')
            str[i] -=7;
        str[i] -= 48;
        if(str[i] >= b)
        {
            state = 1;
            return b;
        }
        res += str[i];
        ++i;
    }
    return res;
}


int main()
{
    std::string str;
    std::cin >> str;
    for(int i = 2; i < 37; ++i)
    {
        state = 0;

        if(base(i, str)%(i - 1) == 0 && !state)
        {
            std::cout << i << std::endl;
            return 0;
        }
    }
    std::cout << "No solution." << std::endl;
    return 0;
}