* {
  box-sizing: border-box;
}

body {
  font-family: Arial, Helvetica, sans-serif;
}


header {
  background-color: #001863;
  padding: 20px;
  text-align: center;
  font-size: 25px;
  color: white;
}


nav {
  float: left;
  width: 10%;
  height: 500px; 
  background: #4e5159;
  padding: 20px;
  color: white;
}


nav ul {
  list-style-type: none;
  padding: 0;
}

article {
  float: left;
  padding: 20px;
  width: 83.99%;
  background-color: #ffffff;
  height: 500px; 
}


section:after {
  content: "";
  display: table;
  clear: both;
}


footer {
  background-color: #001863;
  padding: 60px;
  text-align: center;
  color: white;
}


@media (max-width: 600px) {
  nav, article {
    width: 100%;
    height: auto;
  }
}