How long will it take you to lose a stone if you only eat 1200 calories a day and do exercise to burn 100 calories a day?

ok to lose weight u need to burn more calories then you are consuming.. if you eat 1200 calories and only burn 100 then your total for that day is still +1100 . see what i mean? so you must burn an amount greater then 1200 calories in order to lose weight.. i dotn know how to measure calories burned but i would guess about 2 hours exercise a day would be enough