local function meld(a,b)
    if not a then
        return b
    elseif not b then
        return a
    elseif a[1]<b[1] then
        b[4]=a[3]
        a[3]=b
        return a
    else
        a[4]=b[3]
        b[3]=a
        return b
    end
end

local function pair(a)
    local b=a and a[4]
    if not b then
        return a
    end
    local q={}
    while b do
        local c=b[4]
        a[4]=false
        b[4]=false
        table.insert(q,meld(a,b))
        a=c
        b=a and a[4]
    end
    for i=#q,1,-1 do
        a=meld(a,q[i])
    end
    return a
end


local PairingHeap={}

PairingHeap.empty=function(self)
    return not self[1]
end

PairingHeap.push=function(self,k,v)
	if not self[1] then
		self[1]={
            k,
            v,
            false,
            false,
		}
	else
		if self[1][1]<k then
			self[1][3]={
				k,
				v,
				false,
				self[1][3],
			}
		else
			self[1]={
				k,
				v,
				self[1],
				false,
			}
		end
	end
end

PairingHeap.top=function(self)
    return self[1][2]
end

PairingHeap.pop=function(self)
    self[1]=pair(self[1][3])
end

PairingHeap.join=function(self,other)
	if self[1][1]<other[1][1]then
		other[1][4]=self[1][3]
        self[1][3]=other[1]
	else
		self[1][4]=other[1][3]
        other[1][3]=self[1]
        self[1]=other[1]
    end
    return self
end

PairingHeap.new=function(self)
    local obj={}
    setmetatable(obj,{__index=PairingHeap})
    return obj
end

----------

local n,d,a=io.read("n","n","n")
local pq=PairingHeap:new()
for i=1,n do
    local x,h=io.read("n","n")
    pq:push(x,{x,h,0})
end

local atack=0
local total=0
while not pq:empty() do
    local x,h,c=table.unpack(pq:top())
    pq:pop()
    if c==0 then
        if atack*a<h then
            h=h-atack*a
            local count=math.floor(h/a)+1
            pq:push(x+2*d+0.5,{x+2*d,count,1})
            total=total+count
            atack=atack+count
        end
    else
        atack=atack-h
    end
end
print(total)