N = int(input())
arr = [[0 for _ in range(101)] for _ in range(101)]
for _ in range(N):
a, b = map(int, input().split())
for i in range(a, a+10):
for j in range(b, b+10):
arr[i][j] = 1
count = 0
for c in arr:
count += c.count(1)
print(count)