#include <iostream>

using namespace std;

const int N = 30;
int n, nums[N];
int length;

int binary_search(int nums[],int length, int key) {
    int low = 0, high = length - 1, mid;
    while(low <= high) {
        mid = (low + high) / 2;
        if(nums[mid] == key)
            return mid + 1;
        if(mid < key) 
            low = mid + 1;
        if(mid > key)
            high = mid - 1;
    }
    return -1;
}

int main() {
    cin >> n;
    for (int i = 0; i < n; i ++) {
        cin >> nums[i];
    }
    int res = binary_search(nums, n, 20);
    if(res != -1)
        printf("它在数组中的第%d个位置！", res);
    else 
        printf("对不起，没有此元素！");

    return 0;
}