A = [[0 for _ in range(101)] for _ in range(101)] result = 0 paper_count = int(input()) for _ in range(paper_count): width, height = list(map(int, input().split(' '))) for x in range(width, width+10): for y in range(height, height+10): A[x][y] = 1 for i in A: result += i.count(1) print(result)