import sys
input = sys.stdin.readline
arr = [[0for _ inrange(101)] for _ inrange(101)]
N = int(input())
for _ inrange(N):
x, y = map(int,input().split())
for xidx, X inenumerate(arr):
for yidx, Y inenumerate(X):
if x <= xidx <= x+9and y <= yidx <= y+9 :
arr[xidx][yidx] = 1
answer = 0for i in arr :
answer += sum(i)
print(answer)