#!/usr/bin/env python3

import sys, math, itertools, heapq, collections, bisect, fractions

input = lambda: sys.stdin.buffer.readline().rstrip().decode('utf-8')

sys.setrecursionlimit(10**8)

inf = float('inf')

ans = count = 0



a,b,h=[int(eval(input())) for i in range(3)]

print(((a+b)*h//2))
#!/usr/bin/env python3

import sys, math, itertools, collections, bisect

input = lambda: sys.stdin.buffer.readline().rstrip().decode('utf-8')

inf = float('inf') ;mod = 10**9+7

mans = inf ;ans = 0 ;count = 0 ;pro = 1



a=int(eval(input()))

b=int(eval(input()))

h=int(eval(input()))

print(((a+b)*h//2))
#!/usr/bin/env python3
import sys, math, itertools, heapq, collections, bisect, fractions

input = lambda: sys.stdin.buffer.readline().rstrip().decode("utf-8")
sys.setrecursionlimit(10**8)
inf = float("inf")
ans = count = 0
a, b, h = [int(eval(input())) for i in range(3)]
print(((a + b) * h // 2))

#!/usr/bin/env python3
import sys, math, itertools, collections, bisect

input = lambda: sys.stdin.buffer.readline().rstrip().decode("utf-8")
inf = float("inf")
mod = 10**9 + 7
mans = inf
ans = 0
count = 0
pro = 1
a = int(eval(input()))
b = int(eval(input()))
h = int(eval(input()))
print(((a + b) * h // 2))

-import sys, math, itertools, heapq, collections, bisect, fractions +import sys, math, itertools, collections, bisect -sys.setrecursionlimit(10**8) -ans = count = 0 -a, b, h = [int(eval(input())) for i in range(3)] +mod = 10**9 + 7 +mans = inf +ans = 0 +count = 0 +pro = 1 +a = int(eval(input())) +b = int(eval(input())) +h = int(eval(input()))