Error: assignment of ready-only c++

#include <iostream>
#include "stdlib.h"
#include "time.h"
using namespace std;

int main()
{
    srand((unsigned)time(NULL));
    int X;
    int N=0;
    int Y;
    int A;
    char aller='o';
    const int MAX=999, MIN=0;
    X=rand()%999;
    while (aller='o')
        {
            N=N+1;
            Y = (MAX+MIN)/2;
            if(MAX+MIN & 1)
            {
                A = rand()%1;
                Y=Y+A;
            }
            if (Y>X)
            {
                MAX=Y-1;
                cout << "trop grand";
            }
            aller='o';
            if (Y<X)
            {

MIN=Y+1;
                cout << "trop petit";
            aller='o';
            }

if (Y==X)
            {
                cout << "GAGNER EN : "<<N;
                cout << " COUPS"<<endl;
                return 0;
            }

}
}