* {
  font-family: "ＭＳ Ｐゴシック", "Osaka", sans-serif;
  line-height: 1;
}

html, body {
  width: 100%;
  margin: 0;
  background: url("img/haikei.gif");
  background-size: 40px;
  display: flex;
}

#nav {
  width: 280px;
  height: 100vh;
  border: inset 5px #ccc;
  box-sizing: border-box;
  position: fixed;
}

main {
  text-align: center;
  margin: 8px 0 8px 280px;
  flex-grow: 1;
}

p {
  margin: 4px;
}

@media screen and (max-width: 900px) {
  #nav {
    display: none;
  }

  main {
    margin-left: 0;
  }
}