Why str.length()-1 returning some large integer

The program was throwing error.I figured out that while loop was not terminating because A.size()-1 resulted in some large no even when A.size() has become 0.

I instead used a variable n to store A.size() and used it in while loop it worked.But I want to know why A.size()-1 prints such large no.

    //Input:  A:"nnnn"
    int i=0;
    while(i<A.size()-1){
        if(A[i]==A[i+1]){
            A.erase(A.begin()+i);
            A.erase(A.begin()+i);
            if(i!=0)i--;
        }
        else i++;
    cout<<i<<" "<<A.size()-1<<endl;
    }
    if(A=="")return "empty";
    return A;
    }


    /* //below code works fine
    int i=0;
    int n=A.size();
    while(i<n-1){
        if(A[i]==A[i+1]){
            A.erase(A.begin()+i);
            A.erase(A.begin()+i);
            if(i!=0)i--;
        }
    else i++;
    n=A.size();
    }
    if(A=="")return "empty";
    return A;
    */

    //output
    0 1
    0 18446744073709551615