#include <bits/stdc++.h> using namespace std; int n, a[100005], x; int main() { cin >> n; for (int i=1; i<=n; i++) cin >> a[i]; cin >> x; int l=0, r=n, mid; while (l<r) { int mid = (l+r)/2; if (a[mid] == x) l=mid, r=mid; if (a[mid] < x) l = mid+1; else r = mid-1; } if (a[l] != x || l <= 0 || r > n) { cout << -1; return 0; } cout << l; return 0; }