// editor4
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAX 10000

int main() {
    char str[MAX + 1];
   
    if (fgets(str, sizeof(str), stdin) == NULL) {
        printf("Invalid input\n");
        return 0;
    }
    str[strcspn(str, "\n")] = '\0';
   
    int n = strlen(str);
    if (n < 1 || n > 10000) {
        printf("Invalid input\n");
        return 0;
    }
    for (int i = 0; i < n; i++) {
        if (str[i] < 32 || str[i] > 126) {
            printf("Invalid input\n");
            return 0;
        }
    }
    char stack[MAX];
    int top = -1;
    for (int i = 0; i < n; i++) {
        stack[++top] = str[i];
    }
    while (top >= 0) {
        printf("%c", stack[top--]);
    }
    printf("\n");
   
    return 0;
}



#include <stdio.h>

int isBalanced(int n) {
    int first = n / 100;
    int middle = (n / 10) % 10;
    int last = n % 10;
    return (first + last) == 2 * middle;
}

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 lastBalancedIndex = -1;
    for (int i = 0; i < K; i++) {
        if (isBalanced(capsules[i])) {
            lastBalancedIndex = i;
        }
    }

    if (lastBalancedIndex == -1 || lastBalancedIndex == K - 1) {
        printf("-1\n");
    } else {
        printf("%d\n", capsules[lastBalancedIndex + 1]);
    }