#include <stdio.h>
int main()
{
    long long int a,b,c,d;
    scanf("%lld%lld", &a,&b);
    c = a+b;
    d = c/2;
    if(c%2!=0)
        printf("IMPOSSIBLE\n");

    else

        printf("%lld\n", d);

    return 0;
}
