When two amino acids form a dipeptide, the -NH2 of the former amino acid is combined with the -COOH of the latter amino acid to form a dipeptide and a H2O molecule is removed.

Similarly, in order to form a tripeptide, two water molecules should be removed.

In order to form N peptide, (N- 1) molecular water should be removed.

So the molecular weight of polypeptide or pro is

Sum of molecular weights of amino acids-molecular weight of dehydrated water.