local ior = io.input()
local n = ior:read("*n")
local a = {}
for i = 1, n - 1 do
  a[i] = ior:read("*n")
end
local ret = 0
local hito = 0
for i = 1, n - 1 do
  local b, c = ior:read("*n", "*n")
  hito = hito + c - b
  ret = ret + a[i] * hito
end
ior:read("*n", "*n")  --unused
print(ret)