C++ :

#include<cstdio>
const int N=100;
int x,y;
int f[N];
bool g[N];
int main()
{
	int a,b;
	scanf("%d%d",&x,&y);
	for(int i=1;i<N;i++)
		f[i]=i;
	while(scanf("%d%d",&a,&b)!=EOF)
		f[a]=b;
	g[y]=true;
	while(y!=f[y])
		y=f[y],g[y]=true;
	while(g[x]!=true)
		x=f[x];
	printf("%d\n",x);
	return 0;
}

Pascal :

program p26885;
var f,c:array[1..1000]of integer;
    i,j,k,n,x,y,a,b:integer;
begin
  readln(x,y);
  while not eof do
    begin
      readln(a,b);
      f[a]:=b;
    end;
  i:=x;
  while i<>0 do
    begin
      c[i]:=1;
      i:=f[i];
    end;
  j:=y;
  while c[j]<>1 do
    begin
      j:=f[j];
    end;
  writeln(j);
end.