Aspect ratio changes when importing the footage to the Timeline

hi,

please send me the solution for my problem.

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prob: 

im using after effects 

im importing a file into after effetcs, drag and drop to timeline

every time when i drag and drop to timeline. the file (image/mp4/composition..etc) the size dimensions are changed like hight is increased, width is decreased so its looking ugly. but i need perfect hight,width.

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