from collections import deque

from heapq import heapify,heappop,heappush,heappushpop

from copy import copy,deepcopy

from itertools import product,permutations,combinations,combinations_with_replacement

from collections import defaultdict,Counter

from bisect import bisect_left,bisect_right

# from math import gcd,ceil,floor,factorial

# from fractions import gcd

from functools import reduce

from pprint import pprint



INF = float("inf")



def myinput():

    return list(map(int,input().split()))



def mylistinput(n):

    return [ list(myinput()) for _ in range(n) ]



def mycol(data,col):

    return [ row[col] for row in data ]



def mysort(data,col,reverse_flag):

    data.sort(key=lambda x:x[col],reverse=reverse_flag)

    return data



def mymax(data):

    M = -1*float("inf")

    for i in range(len(data)):

        m = max(data[i])

        M = max(M,m)

    return M



def mymin(data):

    m = float("inf")

    for i in range(len(data)):

        M = min(data[i])

        m = min(m,M)

    return m



def mycount(ls,x):

    # lsはソート済みであること

    l = bisect_left(ls,x)

    r = bisect_right(ls,x)

    return (r-l)



def myoutput(ls,space=True):

    if space:

        if len(ls)==0:

            print(" ")

        elif type(ls[0])==str:

            print((" ".join(ls)))

        elif type(ls[0])==int:

            print((" ".join(map(str,ls))))

        else:

            print("Output Error")

    else:

        if len(ls)==0:

            print("")

        elif type(ls[0])==str:

            print(("".join(ls)))

        elif type(ls[0])==int:

            print(("".join(map(str,ls))))

        else:

            print("Output Error")



a = int(eval(input()))

b = int(eval(input()))

h = int(eval(input()))

ans = (a+b)*h/2

print((int(ans)))
from collections import deque

from heapq import heapify,heappop,heappush,heappushpop

from copy import copy,deepcopy

from itertools import product,permutations,combinations,combinations_with_replacement

from collections import defaultdict,Counter

from bisect import bisect_left,bisect_right

# from math import gcd,ceil,floor,factorial

# from fractions import gcd

from functools import reduce

from pprint import pprint



INF = float("inf")



def myinput():

    return list(map(int,input().split()))



def mylistinput(n):

    return [ list(myinput()) for _ in range(n) ]



def mycol(data,col):

    return [ row[col] for row in data ]



def mysort(data,col,reverse_flag):

    data.sort(key=lambda x:x[col],reverse=reverse_flag)

    return data



def mymax(data):

    M = -1*float("inf")

    for i in range(len(data)):

        m = max(data[i])

        M = max(M,m)

    return M



def mymin(data):

    m = float("inf")

    for i in range(len(data)):

        M = min(data[i])

        m = min(m,M)

    return m



def mycount(ls,x):

    # lsはソート済みであること

    l = bisect_left(ls,x)

    r = bisect_right(ls,x)

    return (r-l)



def myoutput(ls,space=True):

    if space:

        if len(ls)==0:

            print(" ")

        elif type(ls[0])==str:

            print((" ".join(ls)))

        elif type(ls[0])==int:

            print((" ".join(map(str,ls))))

        else:

            print("Output Error")

    else:

        if len(ls)==0:

            print("")

        elif type(ls[0])==str:

            print(("".join(ls)))

        elif type(ls[0])==int:

            print(("".join(map(str,ls))))

        else:

            print("Output Error")



def I():

    return int(eval(input()))



def MI():

    return list(map(int,input().split()))



def RI():

    return list(map(int,input().split()))



def CI(n):

    return [ int(eval(input())) for _ in range(n) ]



def LI(n):

    return [ list(map(int,input().split())) for _ in range(n) ]



def S():

    return eval(input())



def MS():

    return input().split()



def RS():

    return list(input().split())



def CS(n):

    return [ eval(input()) for _ in range(n) ]



def LS(n):

    return [ list(input().split()) for _ in range(n) ]



a = I()

b = I()

h = I()



ans = (a+b)*h//2

print(ans) 

from collections import deque
from heapq import heapify, heappop, heappush, heappushpop
from copy import copy, deepcopy
from itertools import product, permutations, combinations, combinations_with_replacement
from collections import defaultdict, Counter
from bisect import bisect_left, bisect_right

# from math import gcd,ceil,floor,factorial
# from fractions import gcd
from functools import reduce
from pprint import pprint

INF = float("inf")


def myinput():
    return list(map(int, input().split()))


def mylistinput(n):
    return [list(myinput()) for _ in range(n)]


def mycol(data, col):
    return [row[col] for row in data]


def mysort(data, col, reverse_flag):
    data.sort(key=lambda x: x[col], reverse=reverse_flag)
    return data


def mymax(data):
    M = -1 * float("inf")
    for i in range(len(data)):
        m = max(data[i])
        M = max(M, m)
    return M


def mymin(data):
    m = float("inf")
    for i in range(len(data)):
        M = min(data[i])
        m = min(m, M)
    return m


def mycount(ls, x):
    # lsはソート済みであること
    l = bisect_left(ls, x)
    r = bisect_right(ls, x)
    return r - l


def myoutput(ls, space=True):
    if space:
        if len(ls) == 0:
            print(" ")
        elif type(ls[0]) == str:
            print((" ".join(ls)))
        elif type(ls[0]) == int:
            print((" ".join(map(str, ls))))
        else:
            print("Output Error")
    else:
        if len(ls) == 0:
            print("")
        elif type(ls[0]) == str:
            print(("".join(ls)))
        elif type(ls[0]) == int:
            print(("".join(map(str, ls))))
        else:
            print("Output Error")


a = int(eval(input()))
b = int(eval(input()))
h = int(eval(input()))
ans = (a + b) * h / 2
print((int(ans)))

from collections import deque
from heapq import heapify, heappop, heappush, heappushpop
from copy import copy, deepcopy
from itertools import product, permutations, combinations, combinations_with_replacement
from collections import defaultdict, Counter
from bisect import bisect_left, bisect_right

# from math import gcd,ceil,floor,factorial
# from fractions import gcd
from functools import reduce
from pprint import pprint

INF = float("inf")


def myinput():
    return list(map(int, input().split()))


def mylistinput(n):
    return [list(myinput()) for _ in range(n)]


def mycol(data, col):
    return [row[col] for row in data]


def mysort(data, col, reverse_flag):
    data.sort(key=lambda x: x[col], reverse=reverse_flag)
    return data


def mymax(data):
    M = -1 * float("inf")
    for i in range(len(data)):
        m = max(data[i])
        M = max(M, m)
    return M


def mymin(data):
    m = float("inf")
    for i in range(len(data)):
        M = min(data[i])
        m = min(m, M)
    return m


def mycount(ls, x):
    # lsはソート済みであること
    l = bisect_left(ls, x)
    r = bisect_right(ls, x)
    return r - l


def myoutput(ls, space=True):
    if space:
        if len(ls) == 0:
            print(" ")
        elif type(ls[0]) == str:
            print((" ".join(ls)))
        elif type(ls[0]) == int:
            print((" ".join(map(str, ls))))
        else:
            print("Output Error")
    else:
        if len(ls) == 0:
            print("")
        elif type(ls[0]) == str:
            print(("".join(ls)))
        elif type(ls[0]) == int:
            print(("".join(map(str, ls))))
        else:
            print("Output Error")


def I():
    return int(eval(input()))


def MI():
    return list(map(int, input().split()))


def RI():
    return list(map(int, input().split()))


def CI(n):
    return [int(eval(input())) for _ in range(n)]


def LI(n):
    return [list(map(int, input().split())) for _ in range(n)]


def S():
    return eval(input())


def MS():
    return input().split()


def RS():
    return list(input().split())


def CS(n):
    return [eval(input()) for _ in range(n)]


def LS(n):
    return [list(input().split()) for _ in range(n)]


a = I()
b = I()
h = I()
ans = (a + b) * h // 2
print(ans)

-a = int(eval(input())) -b = int(eval(input())) -h = int(eval(input())) -ans = (a + b) * h / 2 -print((int(ans))) +def I(): +    return int(eval(input())) + + +def MI(): +    return list(map(int, input().split())) + + +def RI(): +    return list(map(int, input().split())) + + +def CI(n): +    return [int(eval(input())) for _ in range(n)] + + +def LI(n): +    return [list(map(int, input().split())) for _ in range(n)] + + +def S(): +    return eval(input()) + + +def MS(): +    return input().split() + + +def RS(): +    return list(input().split()) + + +def CS(n): +    return [eval(input()) for _ in range(n)] + + +def LS(n): +    return [list(input().split()) for _ in range(n)] + + +a = I() +b = I() +h = I() +ans = (a + b) * h // 2 +print(ans)