.flex-between {
  display: flex;
  align-items: center;
  justify-content: space-between;
}
.flex-around {
  display: flex;
  align-items: center;
  justify-content: space-around;
}
.flex-center {
  display: flex;
  align-items: center;
  justify-content: center;
}
/* header start */
header {
  width: 100%;
  height: 100px;
}
.nav {
  width: 1200px;
  min-width: 1200px;
  max-width: 1200px;
  height: 100px;
  margin: 0 auto;
}
.logo {
  width: 40px;
  height: 40px;
  margin-right: 20px;
}
.title {
  font-size: 29px;
  font-family: Source Han Sans CN-Bold, Source Han Sans CN;
  font-weight: bold;
  color: #333333;
}
.nav-right > li {
  margin-left: 50px;
}
/* header end */