Bai 1

_data = raw_input()
_data = _data.split()
n = int(_data[0])
if n == 0 or n == 1:
    print -1
else:
    _lst = [int(x) for x in _data[1:]]
    for x in xrange(1,n+1):
        if x not in _lst:
            print x
            break
    else:
        print -1