import sys, itertools
 
sys.setrecursionlimit(500005)
stdin = sys.stdin

ni = lambda: int(ns())
na = lambda: list(map(int, stdin.readline().split()))
ns = lambda: stdin.readline().strip()

n = ni()
L = list()
S = 0

for i in range(n):
    L.append(na())

S += (L[n - 1][0] - L[1][0]) * L[0][1]
S += (L[n - 2][0] - L[0][0]) * L[n-1][1]

for i in range(1, (n- 1)):
    S += (L[i - 1][0] - L[i + 1][0]) * L[i][1]

print(S)