function updateCount(operand) {
var params = {
    TableName : "counter",
    Key:{
        "type" : {
            "S" : "student"
        }
    },
    UpdateExpression : "SET pos = pos + :o",
    ExpressionAttributeValues:{
        ":o": Number(operand)
    },
    ReturnValues:"UPDATED_NEW"
}
docClient.update(params, function(err, data) {
if (err)
        console.log(err);
    else {
        console.log(data);
        res.send(JSON.stringify(data, null, 2));
    }
});

ExpressionAttributeValues:{
    ":o": {"N": operand}
},