import sys
import math as mas

n=int(input())
sum=0
for i in range(n):
	s=input()
	a,b=map(int,s.split())
	sum+=b*((a+1)//2)
print(sum)
#for i in sys.stdin:
#	a,b=map(int,i.split())
#	print(gcd(a,b),lcm(a,b))