1 #define mm(a) memset(a,0,sizeof(a));
 2 #define max(x,y) (x)>(y)?(x):(y)
 3 #define min(x,y) (x)<(y)?(x):(y)
 4 #define Fopen freopen("1.in","r",stdin); freopen("m.out","w",stdout);
 5 #define rep(i,a,b) for(int i=(a);i<=(b);i++)
 6 #define per(i,b,a) for(int i=(b);i>=(a);i--)
 7 #include<bits/stdc++.h>
 8 typedef long long ll;
 9 #define PII pair<ll,ll>
10 using namespace std;
11 const int INF=0x3f3f3f3f;
12 const int MAXN=(int)2e5 + 5;
13 const ll mod=1e9+7;
14 
15 
16 string input,temp;
17 vector<int>v;
18 map<int,int>mp;
19 int n,dp[MAXN],m;
20 int main() {
21     cin>>input;
22     istringstream iss(input);
23     mp.clear();
24     v.clear();
25     v.push_back(0);
26     while (getline(iss, temp, ',')) {
27         int x=atoi(temp.c_str());
28         v.push_back(x);
29         mp[x]++;
30     }
31     n=v.size()-1;
32     int ans;
33     for(auto i:mp) {
34         if(i.second==1)ans=i.first;
35     }
36     printf("%d\n",ans);
37     return 0;
38 }