C++: Data Structure / queue

I made a data structure algorithm. I put all my knowledge into it. I can't find the error. I leave the statement of the exercise below:

Make a program that creates two rows F1 and F2, of size 10, each. Read 20 integers, and if the Read number is even, enter it in the queue F1 and, if odd, in row F2. In the sequence, if the number Read is multiple of 3, remove an element from the F1 queue, storing it in a auxiliary variable and then write it in the video and enter it in the F2 queue. If the Read number is multiple of 5, remove an element from queue F2, storing it in an auxiliary variable and then writing it to the video and enter it in the F1 queue. A number can be both multiple of 3 and multiple of 5 and in this case, present a message in the video and do nothing in the queues, moving on to the reading of the next number. Stop reading when have already read the 20 numbers or when overflow occurs or underflow in some line.

int main()
{
    int f=-1, r=-1, f1[10], f2[10], cont=0, val=0, aux=0, tam=9;

do{
    cout << "Informe um valor: " << endl;
        cin >> val;
    cout << " " << endl;

    if(val%2==0){
            if(r==tam){
                cout << "OVERFLOW!" << endl << endl;
                    return 0;
            }else{
                f1[++r]=val;
            }
    }else{
        if(r==tam){
            cout << "OVERFLOW!" << endl << endl;
                return 0;
        }else{
                f2[++r]=val;
        }
    }

    if(val%3==0){
            if(r==f){
                cout << "UNDERFLOW!" << endl << endl;
                    return 0;
            }else{
            aux=f1[f+1];
            f++;
            if(f==r){
                f=r=-1;
            cout << "Valor retirado: " << aux << endl << endl;
            f2[++r]=aux;
            }
            }
    }

    if(val%5==0){
            if(r==f){
                cout << "UNDERFLOW!" << endl << endl;
                    return 0;
            }else{
            aux=f2[f+1];
            f++;
            if(f==r){
                f=r=-1;
            cout << "Valor retirado: " << aux << endl << endl;
            f1[++r]=aux;
            }
            }
    }

    if(val%3==0 && val%5==0){
        cout << "Numero mulitplo de 3 e 5!" << endl << endl;
    }

cont++;
}while(cont < 20);
return 0;
}