Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
Tell me the sum of cars per set for operator of london midland

### Inputs:
table_name: table, columns: ['Class', 'Operator', 'No. Built', 'Year Built', 'Cars per Set']

### Response:
SELECT SUM Cars per Set FROM table WHERE Operator = london midland