* {
    box-sizing: border-box;
}

body {
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    font-family: Arial, Helvetica, sans-serif;
}

header {
    width: 100%;
    background: #777;
    color: white;
    padding: 15px;
}

h1 {
    margin: 0;
    font-family: Georgia, 'Times New Roman', Times, serif;
}

main {
    display: flex;
    flex: 1;
}

nav {
    width: 250px;
    padding: 10px;
}

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

nav li {
    background: #808080;
    padding: 10px;
    margin-bottom: 4px;
}

nav a {
    display: block;
    text-decoration: none;
    color: black;
    font-weight: bold;
    padding: 5px;
}

nav li:hover {
    background: #a7a7a7;
}

footer {
    width: 100%;
    background: #777;
    padding: 10px;
    margin: 0;
    color: white;
}

#principal {
    flex: 1;
    border-left: 2px solid black;
    padding: 20px;
}

#principal h2 {
    margin-top: 0;
}