// editor5
#include <stdio.h>
#include <stdlib.h>

int main() {
    int K;
    if (scanf("%d", &K) != 1 || K < 4 || K > 10) {
        printf("Invalid input\n");
        return 0;
    }

    int capsules[K];
    for (int i = 0; i < K; i++) {
        if (scanf("%d", &capsules[i]) != 1 || capsules[i] < 100 || capsules[i] > 999) {
            printf("Invalid input\n");
            return 0;
        }
    }

    int last_balanced_index = -1;

    for (int i = 0; i < K; i++) {
        int val = capsules[i];
        int first_digit = val / 100;
        int last_digit = val % 10;
        int middle_digit = (val / 10) % 10;

        if ((first_digit + last_digit) == 2 * middle_digit) {
            last_balanced_index = i;
        }
    }

    if (last_balanced_index == -1) {
        printf("-1\n");
    } else if (last_balanced_index == K - 1) {
        // No capsule after the last balanced one
        printf("-1\n");
    } else {
        printf("%d\n", capsules[last_balanced_index + 1]);
    }

    return 0;
}

=============================================================================================================================================================================================

stack section 2 
workout 2

// editor2
#include <stdio.h>
#include <ctype.h>

#define MAX 100

int stack[MAX];
int top = -1;

int main() {
    int n;
    scanf("%d", &n);
    while (n--) {
        char exp[MAX];
        scanf("%s", exp);

        top = -1;
        int valid = 1;

        for (int i = 0; exp[i]; i++) {
            char c = exp[i];
            if (isdigit(c)) {
                stack[++top] = c - '0';
            } else if (c == '+' || c == '-' || c == '*' || c == '/') {
                if (top < 1) {
                    valid = 0;
                    break;
                }
                int b = stack[top--];
                int a = stack[top--];
                if (c == '+') stack[++top] = a + b;
                else if (c == '-') stack[++top] = a - b;
                else if (c == '*') stack[++top] = a * b;
                else {
                    if (b == 0) {
                        valid = 0;
                        break;
                    }
                    stack[++top] = a / b;
                }
            } else {
                valid = 0;
                break;
            }
        }

        if (valid && top == 0)
            printf("%d\n", stack[top--]);
        else
            printf("Invalid input\n");
    }
    return 0;
}