Adalea: on the eastern border (border of Poland and Russia)
Adalea: looking on a map it seems that the border between poland and russia was at the north-eastern most part of poland... because the eastern section was bordering Ukraine & Lithuania I believe.
Only if you are looking at today's maps :) Between world wars (1918-1939) the borders of Poland were much more towards East, but the Soviet Union took them (about 40% of the pre-war Poland) after "liberating" us from Germans in 1945 (those decisions about borders were made in cooperation with the Allies and they gave us some lands in the West, previously ruled by the Germans, like Wrocław/Breslau and Szczecin/Stettin).
If your father was talking about the times before they had left Poland (around 1930) he probably meant Russian border in the East (Ukraine as a indipendent country has been existing only since 1991, for many centuries its lands were ruled by Poland, Polish-Lithuanian Commonwealth, then Russia and finally Soviet Union).
Here's a map of Poland in 1918-1939:

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