def requiredArea(n, x, y):
    covered = set()
    
    for i in range(n):
        w, h = x[i], y[i]
        for dx in range(-w//2, w//2 + 1):
            for dy in range(-h//2, h//2 + 1):
                covered.add((dx, dy))
    
    return len(covered)

def main():
    n = int(input())
    x, y = [], []
    
    for i in range(n):
        s = input().split()
        x.append(int(s[0]))
        y.append(int(s[1]))
    
    print(requiredArea(n, x, y))

main()