def Area(u,v,w):return(v[0]-u[0])*(w[1]-u[1])-(v[1]-u[1])*(w[0]-u[0])
def sign(n):return[-1,1][n>0]if n else 0
def L1(v):return abs(v[0])+abs(v[1])
def L1_Distance(u,v):return L1([u[0]-v[0],u[1]-v[1]])
def L1_LongestEdge(u,v,w):return L1_Distance(u,v)>=max(L1_Distance(u,w),L1_Distance(w,v))
def IntersectionType(u,v,z,w):s=[sign(Area(u,v,z)),sign(Area(u,v,w)),sign(Area(z,w,u)),sign(Area(z,w,v))];return 1 if s[0]*s[1]<0 and s[2]*s[3]<0 else 0 if(s[0]==0 and L1_LongestEdge(u,v,z))or(s[1]==0 and L1_LongestEdge(u,v,w))or(s[2]==0 and L1_LongestEdge(z,w,u))or(s[3]==0 and L1_LongestEdge(z,w,v))else -1
R=range
J=lambda:list(map(int,input().split()))
N,*_=J()
XY=[J()for i in R(N)]
answer=-1
def xy(i):return XY[i if i<N else i-N]
def intersection_type(i,j):return IntersectionType(xy(i),xy(i+1),xy(j),xy(j+1))
def area_sign(i,j,k):return sign(Area(xy(i),xy(j),xy(k)))
for i in R(N):
	s=area_sign(i,i+1,i+2)
	if s==0:exit(print(0))
	for j in R(2,N):
		if j<N-1 and intersection_type(i,i+j)!=-1:exit(print(0))
		t=area_sign(i,i+j-1,i+j)
		if s*t<0:s=0;break
	if s!=0:answer=1
print(answer)