modify formula whiel copying to below cell

i have totals in Column 'C' on every 6th row on Sheet1
ie: totals in cells C7, C13, C19, C25. here the difference is always SIX

i am refering to C7 in Sheet2 saying Total Visit Week 1 - =Sheet1!C7

when i copy the formula to below cell ie:Total Visit Week 2 it should change to =Sheet1!C13, =Sheet1!C19 and =Sheet1!C25 onwards.

attached file shows sample data.

Please help out!

Thanks !