H, W = map(int, input().split())
p = []
for _ in range(W):
	x, y = map(int, input().split())
	p.append((-x + 2 * y, x, y))
p.sort()
ans = 0
for i in range(W):
	x, y = p[i][1] - 1, p[i][2] - (i + 1)
	if x * y >= 0:
		ans += max(abs(x), abs(y))
	else:
		ans += abs(x) + abs(y)
print(ans)