A
   ]  
   |
   |_________________
   D                                |  B
                                      |
                                      |
                                       C
Let ABCD be this parallelogram 
AB + BC + CD + DA = 50, but AD = BC  and AB = DC, Then:
2.AD + 2.AB = 50 and :
AD+AB = 25 . We also know that AB+1 = AD (Given), then:
AD + (AD+1) = 25
2.AD = 24 
AD = 12, THEN AB = 12+1=13
Calculate DB the diagonal perpendicular to AD, using Pythagoras:
AD² + DB² = AB²
12² +DB² = 13²
DB² = 13² - 12² = 25
AND DB= 5