c++ exception, don't know how to fix

I want to write a log in feature in C++. I've written the code, but when I tried to compile it it just crashed. So I started some debugging and got the following error: Exception triggered at 0x7A055378 (vcruntime140d.dll) in Einkaufsladen.exe: 0xC0000005: Access violation when reading at position 0xDDDDDDDD.

for (int i = menge; i >= 0; i--)
    {
        if ( gesamtmenge[k].Name == gesamtmenge[i].Name ) // here I get the error
        {
            if (gesamtmenge[k].Passwort == gesamtmenge[i].Passwort)
            {
                return true;
                i = -1;
            }
        }
        else
        {
            return false;
        }
    }
    f.close();

This of course isn't the whole code but it hould be the necessary part. If anything more is needed just say so.

I'm greatful for any help.