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

int solution(int* arr, int n) {
    // Calculate common difference
    int commonDiff = (arr[n - 1] - arr[0]) / n;
    
    // Binary search for the position where arr[i] != arr[0] + i * diff
    int left = 0, right = n - 1;
    
    while (left <= right) {
        int mid = left + (right - left) / 2;
        int expectedValue = arr[0] + mid * commonDiff;
        
        if (arr[mid] == expectedValue) {
            // Missing element is to the right
            left = mid + 1;
        } else {
            // Missing element is at or to the left of mid
            right = mid - 1;
        }
    }
    
    // The missing element should be at position 'left'
    return arr[0] + left * commonDiff;
}

void parseArray(const char* str, int* arr, int* size) {
    *size = 0;
    const char* p = str;
    while (*p && *p != '[') p++;
    if (*p == '[') p++;
    while (*p && *p != ']') {
        while (*p == ' ' || *p == ',') p++;
        if (*p == ']' || *p == '\0') break;
        arr[(*size)++] = (int)strtol(p, (char**)&p, 10);
    }
}

int main() {
    char line[1000];
    fgets(line, sizeof(line), stdin);
    
    // Parse JSON array manually
    int arr[100];
    int n = 0;
    char* token = strtok(line + 1, ",]"); // Skip opening bracket
    
    while (token != NULL) {
        arr[n++] = atoi(token);
        token = strtok(NULL, ",]");
    }
    
    int result = solution(arr, n);
    printf("%d\n", result);
    
    return 0;
}