import std.algorithm, std.container, std.conv, std.math, std.range, std.typecons, std.stdio, std.string;

auto rdsp(){return readln.splitter;}
void pick(R,T)(ref R r,ref T t){t=r.front.to!T;r.popFront;}
void readC(T...)(size_t n,ref T t){foreach(ref v;t)v=new typeof(v)(n);foreach(i;0..n){auto r=rdsp;foreach(ref v;t)pick(r,v[i]);}}

void main()
{
  string[] s; readC(3, s);

  auto t = 0, i = new int[](3);
  for (;;) {
    if (i[t] >= s[t].length) {
      writeln(cast(char)('A'+t));
      return;
    }
    t = cast(int)(s[t][i[t]++]-'a');
  }
}
