n , k = map(int,input().split())
a = list(map(int,input().split()))
l = a[k-1]
for i in a[k:]:
if i != l:
print(-1)
break
else:
for i,val in enumerate(reversed(a[:k-1])):
if val != l:
print(k-i-1)
break
else:
print(0)
#include<bits/stdc++.h>
using namespace std;
int a[100005],n,k,ans;
int main()
{
cin>>n>>k;
for(int i=1;i<=n;i++)
{
cin>>a[i];
}
for(int i=k;i<=n;i++)
{
if(a[i]!=a[k])
{
cout<<-1<<endl;
return 0;
}
}
for(int i=1;i<k;i++)
{
if(a[i]!=a[k])
{
ans=i;
}
}
cout<<ans<<endl;
return 0;
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