how to count the number

Hello everyone, I am a newbie
in collections,I create a c.ts,it contain C1,C2,C3,Now I wanna make c3=c1+c2,how to get c1,c2 value in c3's hook, pls help to how to do it,thanks，
my code is below:
`
import { CollectionConfig, Field } from 'payload/types';

const Tags: CollectionConfig = {
slug: 'c_sum',

admin: {
useAsTitle: 'name',
defaultColumns: ['name', 'c1','c2','c_sum']
},
access: {
read: () => true,
create:()=>true,
},
fields: [
{
type: 'text',
name: 'name',
},
{
type: 'number',
name: 'c1',
label: "c1",
},
{
type: 'number',
name: 'c2',
label: "c2",
},
{
type: 'number',
name: 'c_sum',
label: "c_sum",
},

],
timestamps: false,
}
export default Tags;`