#!/usr/bin/bc

#
# See https://theweeklychallenge.org/blog/perl-weekly-challenge-151
#

#
# Run as: bc ch-2.bc < input-file
#
# End each line of input with a 0
# End input with a -1

while (1) {
    n = read ()
    if (n < 0) {
        break
    }
    sz = 0
    while (n > 0) {
        h [sz] = n
        sz = sz + 1
        n = read ()
    }
    h [sz + 0] = 0
    h [sz + 1] = 0
    for (i = sz - i; i >= 2; i --) {
        if (h [i] + h [i + 2] > h [i + 1]) {
            h [i] = h [i] + h [i + 2]
        } else {
            h [i] = h [i + 1]
        }
    }
    h [0] + h [2]
}


halt