#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

#define MAXN 100100
#define EPS 0.000001

int compare (const void * a, const void * b) {
    if ( *(double*)a <  *(double*)b )
        return -1;
    else if ( *(double*)a == *(double*)b )
        return 0;
    else
        return 1;
}

double area( double vec[MAXN], int n, double cut ) {

    int i;
    double a = 0.0;

    for ( i = 0; i < n; i++ ) {
        if ( cut < vec[i] )
            a += vec[i]-cut;
    }

    return a;
}

int main() {

    double H[MAXN], a;
    int n;

    while ( scanf("%d%lf", &n, &a), n != 0 ) {

        bool done = false;
        int i, h;
        double areaTotal = 0;

        for ( i = 0; i < n; i++ ) {
            scanf("%lf", &h);
            H[i] = h;
            areaTotal += h;
        }

        if ( areaTotal == a ) {
            printf(":D\n");
            continue;
        }
        else if ( areaTotal < a ) {
            printf("-.-\n");
            continue;
        }

        qsort( H, n, sizeof(int), compare );

        double cut;
        double start = H[0], end = H[n-1];

        while ( !done ) {

            cut = ( start + end )/2;
            double cutArea = area( H, n, cut );

            if ( cutArea < a ) {
                start = cut;
            }
            else if ( cutArea > a ){
                end = cut;
            }
            else if ( (end - start) < EPS ) {
                done = true;
            }
        }

        printf("%.4lf\n", cut);
    }

    return 0;
}