import sys
def debug(*args, **kwd):
    import os
    if os.getenv('DEBUG'): print(*args, **kwd, file=sys.stderr)

def input():
    return sys.stdin.readline().rstrip()

def main(_=0):
    M, D = map(int, input().split())
    if (M, D) == (4, 1):
        print(-1)
    else:
        print(M + D)

def as_input(s: str) -> None:
    import io
    global input
    f = io.StringIO(s)
    input = lambda: f.readline().rstrip()
    return None

sample1 = """7 7
"""
sample2 = """12 25
"""
sample3 = """1 1
"""
def test():
    """
    >>> main(as_input(sample1))
    14
    >>> main(as_input(sample2))
    37
    >>> main(as_input(sample3))
    2
    """
    pass

if __name__ == '__main__':
    main()