Why does a vector value stored in an integer give different results?

I have very recently started using vectors in C++.The purpose of the program I am trying to write is to determine the first missing positive number. Given an input array A : [ -9, 12, -1, 0, 1 ] the expected output is 2. I coded it -

int Solution::firstMissingPositive(vector<int> &A)
{
    std::sort(A.begin(),A.end());    //sorting vector

    bool Negative=false;
    int flag;
    int i=0;

    for(i=0;i<A.size();++i)
    {
        if(A[i]<=0)
        {
            Negative=true;
            std::cout<<"\nwe found a -ve number or 0";
        }
        else if( (A[i]>0) && (A[i+1]!=A[i]+1) )
        {
            Negative=false;
            std::cout<<"\ncomparing @ ="<<i<<" which = "<<A[i];
            std::cout<<"\ncomparing @ ="<<i+1<<" which = "<<A[i+1];

            flag=A[i]+1; //The faulty statement

            std::cout<<"\n\n missing number(A[i]+1) @ ="<<A[i]+1;
            std::cout<<"\n\n missing number(flag) @ ="<<flag;
            break;
        }

    }
//do something more
}

With this output -

-9 -1 0 1 12 
we found a -ve number or 0
we found a -ve number or 0
we found a -ve number or 0
comparing @ =3 which = 1
comparing @ =4 which = 12

missing number(A[i]+1) @ =2

missing number(flag) @ =20

I found this interesting because, to me, it looks like I cannot use an integer to store the value of a vector.

• Trying to debug it I found changing the flag assignment to flag = A[i]+2 makes the resultant print 30.
• I've read other questions on SO where it suggests using vector.at(i) instead of the [] operator as a better practice. Changing this does not reflect any change for my code.
• Changing flag to vector<int> gives me a dirty error which I'm not sure about.

Isn't A[i]+1 syntactically equivalent to an integer value? And if it is, why can I not store it?