#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i,n) for(int i=0;i<n;i++)
#define int long long
string a[4];
int n[3];
signed main(void) {
  getline(cin,a[0]);
  getline(cin,a[1]);
  getline(cin,a[2]);
  getline(cin,a[3]);
  if(a[2] == ""){
    cout<<"assert"<<endl;
    return 0;
  }
  n[0]=a[1].at(0)-'0';
  n[1]=a[2].at(0)-'0';  
  n[2]=a[3].at(0)-'0'; 
  //cout<<n[0]<<" "<<n[1]<<" "<<n[2]<<endl;
  map<int,int>mp;
  mp[n[0]+n[1]]++;
  mp[n[2]+n[1]]++;
  mp[n[0]+n[2]]++;   
  int cnt=1;
  for(auto e:mp){
    cnt++;
    if(cnt==2){
      cout<<e.first<<endl;
      return 0;
    }
  }
}