n=int(input());a=[]
for i in range(n+1):
    k=int(input())
    a.append(k)
i=0;j=len(a)-2
while i<=j:
    m=(i+j)//2
    if a[-1]<=a[m]:
        j=m-1
    else:
        i=m+1
if i!=n and a[-1]==a[i]:
    print(i+1)
else:
    print(-1)