Subtract a value or another variable to your variable. Pass an integer or the name of a second variable as the parameter. Also supports - as shorthand.
capacity: 2500reservations: 1900{{ capacity | subtract(1900) }}{{ capacity | subtract($reservations) }}{{ capacity | -($reservations) }}{{ Statamic::modify($capacity)->subtract(1900) }}{{ Statamic::modify($capacity)->subtract($reservations) }}600600600
