int function_recursion(int length, int i)
{
    if (length == 0) return amount;
    if (str[i] == '0' || str[i] == '1' || str[i] == '2' || str[i] == '3' || str[i] == '4' 
        || str[i] == '5' || str[i] == '6' || str[i] == '7' || str[i] == '8' || str[i] == '9') amount++;
    i++; 
    length--;
    return function_recursion(length, i);
}

if (str[i] == '0' || str[i] == '1' || str[i] == '2' || str[i] == '3' || str[i] == '4' 
        || str[i] == '5' || str[i] == '6' || str[i] == '7' || str[i] == '8' || str[i] == '9') amount++;