#include <iostream>
#include <cstdio>
#include <cstring>

using namespace std;

int main()
{
    int a,b;

    while(~scanf("%d%d",&a,&b)){

        if((double)(a+b)/2-(double)((a+b)/2)>0)
            printf("IMPOSSIBLE\n");
        else
            printf("%d\n",(a+b)/2);
    }
    return 0;
}

