简简单单

#include <bits/stdc++.h>
using namespace std;

int n, a[100001];

int main() {
	scanf("%d", &n);
	for (int i = 1; i <= n; i ++)
		scanf("%d", &a[i]);
    int x;
    scanf("%d", &x);
    int L = 0, R = n + 1;
    while (L + 1 < R) {
        int M = (L + R) / 2;
        if (a[M] < x)
            L = M;
        else
            R = M;
    }
    if (R <= n && a[R] == x)
        printf("%d\n", R);
    else
        printf("-1\n");			
}