#sum {
    font-size: xx-large;
    color: red;
    text-align: center;
}
#summand2 {
    font-size: xx-large;
    text-align: center;
}
#summand1 {
    text-align: right;
}
button {
    font-size: xx-large;
    display: block;
    margin: auto;
}
label {
    margin-left: auto;
    margin-right: auto;
}
form {
  text-align: center;
}
div#problem {
    background-image: linear-gradient(135deg, white 50%, olive 100%);
    background-repeat: no-repeat;
    box-shadow: 0 4px 8px 0 rgba(0,0,0,0.2);
    border-left: 10px solid Gainsboro;
    border-left: 3px solid Gainsboro;
    position: relative;
    margin: 1.5em 10px;
    padding: 0.5em 10px;
}