−−−−−

＋−−−−
−＋−−−
−−＋−−
−−−＋−
−−−−＋

＋−−−＋
−＋−−＋
−−＋−＋
−−−＋＋

＋−−＋＋
−＋−＋＋
−−＋＋＋

＋−＋＋＋
−＋＋＋＋

＋＋＋＋＋

１＋５＋４＋３＋２＋１＝１＋５（５＋１）／２。