풀이
N = int(input())
board = [[0 for _ in range(100)] for _ in range(100)]
result = 0
for _ in range(N):
x, y = map(int, input().split())
for i in range(y, y+10):
for j in range(x, x+10):
if board[i][j] == 0:
board[i][j] = 1
result += 1
print(result)