Add calculated column

[msniha]

I have a pivot table that display reference cost by week, like above :
<code style="margin: 0px; padding: 0px; border: 0px; font-family: Consolas, Menlo, Monaco, 'Lucida Console', 'Liberation Mono', 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', 'Courier New', monospace, sans-serif; white-space: inherit;">Ref 201450 201451 201452 20152 20153 20154 20155
0267_C 37,48 37,48
0712_B 29,98
0880 1,25
0931_B 23,43 23,43
0941_D 27,02
0956_E 27,48 29,98
0959_A 6,25
0970_B 33,03 5,00
0993_E 74,96
2016 24,31
2151 1,25
2221_C 72,92
2228_C 52,65 8,75
</code>i want to add a calculated column that fill the blank column with the first week it's produced and then with the last time if it's produced another time The result should be like above ;
<code style="margin: 0px; padding: 0px; border: 0px; font-family: Consolas, Menlo, Monaco, 'Lucida Console', 'Liberation Mono', 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', 'Courier New', monospace, sans-serif; white-space: inherit;">Ref 201450 201451 201452 20152 20153 20154 20155
0267_C 37,48 37,48 37,48 37,48 37,48 37,48 37,48
0712_B 29,98 29,98 29,98 29,98 29,98 29,98 29,98
0880 1,25 1,25 1,25 1,25 1,25 1,25 1,25
0931_B 23,43 23,43 23,43 23,43 23,43 23,43 23,43
0941_D 27,02 27,02 27,02 27,02 27,02 27,02 27,02
0956_E 27,48 27,48 27,48 27,48 27,48 27,48 29,98
0959_A 6,25 6,25 6,25 6,25 6,25 6,25 6,25
0970_B 33,03 5,00 5,00 5,00 5,00 5,00 5,00
0993_E 74,96 74,96 74,96 74,96 74,96 74,96 74,96
2016 24,31 24,31 24,31 24,31 24,31 24,31 24,31
2151 1,25 1,25 1,25 1,25 1,25 1,25 1,25
2221_C 72,92 72,92 72,92 72,92 72,92 72,92 72,92
2228_C 52,65 52,65 52,65 52,65 52,65 52,65 8,75
</code>Thanks.