numOfDragon.js

//Give level and quantity of dragon you want to merge
//Calculate number of level 1 dragon you need to reach it.
//Knowning merging 3 lower level dragons will get 1 dragon which has higher 1 level.
//Merging 5 lower level will get 2 dragons which have higher 1 level.
function numOfDragon(level, quantity) {
    if(level == 0) return 0;
    else if(level == 1) return 1 * quantity;
    if(quantity % 2 == 0) {
        return numOfDragon(level - 1, quantity / 2 * 5);
    }
    else {
        return numOfDragon(level - 1, (quantity - 1) / 2 * 5 + 3);
    }
}