색종이

import sys


def main():
    input = sys.stdin.readline

    n = int(input())
    paper = [[0] * 100 for _ in range(100)]

    for _ in range(n):
        x, y = map(int, input().split())
        for i in range(x, x + 10):
            for j in range(y, y + 10):
                paper[i][j] = 1

    cnt = 0
    for i in range(100):
        for j in range(100):
            if paper[i][j] == 1:
                cnt += 1

    print(cnt)


main()