(n,),*e=[[*map(int,s.split())]for s in open(0)]
INF=1<<60
dp=[[-INF,-INF]for _ in range(n+1)]
dp[0]=0,0
bx=by=-1
for i in range(1,n+1):
  x,y=e[i-1]
  dp[i][0]=max(dp[i-1][0]+(by==x)*x,dp[i-1][1]+(bx==x)*x)+(x==y)*x
  dp[i][1]=max(dp[i-1][0]+(by==y)*y,dp[i-1][1]+(bx==y)*y)+(x==y)*x
  bx,by=x,y
print(max(dp[n]))