# Arithmetische Reihe

a = float(input('Anfangsglied a = '))
d = float(input('Differenz    d = '))
n = int(input('Anzahl der Summanden n = '))

sum=a
for i in range(1,n):
    sum = sum + (a + i*d)

print(a,' + ', a+d,' + ',a+2*d,' + . . . . + ',a+(n-1)*d,' = ',sum)