Parameters_JSON

"Parameters" : {
      "KeyName" : {
      "Description" : "Name of an existing EC2 KeyPair to enable SSH access to the instances",
      "Type" : "String",
      "MinLength": "1",
      "MaxLength": "64",
      "AllowedPattern" : "[-_ a-zA-Z0-9]*",
      "ConstraintDescription" : "can contain only alphanumeric characters, spaces, dashes and underscores."
    },

    "VpcId" : {
      "Type" : "String",
      "Description" : "VpcId of your existing Virtual Private Cloud (VPC)"
    },

    "SubnetId" : {
      "Type" : "String",
      "Description" : "SubnetId of an existing Public facing subnet in your Virtual Private Cloud (VPC)"
    }
  },