#include <iostream>
using namespace std;
int n, k, a[100005];
int main(){
cin >> n >> k;
for (int i = 1; i <= n; i++){
cin >> a[i];
}
int t = a[n];
int z = 0;
for (int i = n-1; i >= 1; i--){
if (a[i]!=t){
z = i;
//cout << t << endl;
break;
}
}
if (k>z) cout << z;
else cout << -1;
return 0;
}
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