How to set up an cloudwatch alarm for lambda timeout?

I have a lambda created via cloudformation template, as such. I have set my memory size and timeout period. I would like to fire off an alarm , if lambda times out. I am know sure what metric name, statistics and other values to set for my alarm below

Resources: 
  someLambda:
    Type: AWS::Serverless::Function
    Properties:
      FunctionName: "MyFunct"
      Description: "..."
      MemorySize: 512
      Timeout: 500
   
 timeoutAlarm:
    Type: AWS::CloudWatch::Alarm
    Properties:
      AlarmName: "timeout"
      AlarmDescription: "Alarm if lambda times out"
      MetricName: ...
      Dimensions:
      - Name: "FunctionName"
        Value: MyFunct
      Statistic: ...