n,p,k=map(int,input().split())
x=10**18
q=[-1]*(k+1)
q[0]=p
g=-1
for _ in range(n):
	t,b=map(int,input().split())
	for i in reversed(range(k)):
		if q[i]!=-1:
			tt=t*a[i]+b
			if tt<=x:
				q[i+1]=max(q[i+1],tt)
	if q[k]<=x:
		g=max(g,q[k])
print(g)