2021/05/06
import sys
N = int(sys.stdin.readline())
li = []
for _ in range(N):
tmp = list(map(int, sys.stdin.readline().split()))
li.append(tmp)
g_map = [[0 for _ in range(100)] for _ in range(100)]
for item in li:
y, x = item
for i in range(y, y + 10):
for j in range(x, x + 10):
if (not g_map[i][j]):
g_map[i][j] = 1
cnt = 0
for i in range(100):
for j in range(100):
if (g_map[i][j]):
cnt+=1
print(cnt)