Q:

invoice amount is 1200000 but I paid 1201035.  What is 1035? 

I paid the invoice and then I noticed I paid more than the amount of the invoice. 
1201035 (I actually paid) - 1200000 (the amount of the invoice) = 1035 (?) 

My questions are..

What is 1035 satoshi? ( routing fee..?) 
Is it possible to know how much extra amount I will pay before paying. 

$ lightning-cli pay lntb12u1pw0ljy4pp55mjg93fckm3ydxsdglp4xxtc8vuc0v79g24gm3rz8h72j8fg32tqdp2xys9xct5da3kx6twv9kk7m3qg3hkccm9ypxxzar5v5cqp5336eqcvsmkx9507p6r82ekralj82jl3npv0v2k8auj2kkgmc9l68mmmzkxvpcnzd3feyygp8hw6wq2qup6f97zfjlm2hzhlk4a9veegqstahde
{
   "id" : 1,
   "payment_hash" : "a6e482c538b6e2469a0d47c35319783b3987b3c542aa8dc4623dfca91d288a96",
   "destination" : "03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134",
   "msatoshi" : 1200000,
   "amount_msat" : "1200000msat",
   "msatoshi_sent" : 1201035,
   "amount_sent_msat" : "1201035msat",
   "created_at" : 1560268918,
   "status" : "complete",
   "payment_preimage" : "c718e470dcf429280c611afba095c7396e219d60e590c6ee6beca8042f616483",
   "bolt11" : "lntb12u1pw0ljy4pp55mjg93fckm3ydxsdglp4xxtc8vuc0v79g24gm3rz8h72j8fg32tqdp2xys9xct5da3kx6twv9kk7m3qg3hkccm9ypxxzar5v5cqp5336eqcvsmkx9507p6r82ekralj82jl3npv0v2k8auj2kkgmc9l68mmmzkxvpcnzd3feyygp8hw6wq2qup6f97zfjlm2hzhlk4a9veegqstahde"
}

$ lightning-cli listpeers
{
   "peers" : [
      {
         "id" : "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248",
         "connected" : true,
         "netaddr" : [
            "23.237.77.12:9735"
         ],
         "globalfeatures" : "",
         "localfeatures" : "82",
         "channels" : [
            {
               "state" : "CHANNELD_NORMAL",
               "scratch_txid" : "e7a65a4f1e625a218baf640a154b04bdf06aa922ff1970ee64cbb3f0046b7866",
               "owner" : "lightning_channeld",
               "short_channel_id" : "1542376x20x0",
               "direction" : 1,
               "channel_id" : "6673cb8effd2bae6e0168949fcd8581acd1f6149f0966b568e8900d384f0729c",
               "funding_txid" : "9c72f084d300898e566b96f049611fcd1a58d8fc498916e0e6bad2ff8ecb7366",
               "private" : false,
               "funding_allocation_msat" : {
                  "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248" : 0,
                  "023a41771d55140a27c35687727a4b51bc6b58254129bd216fb000f405b7701cb4" : 20000000
               },
               "funding_msat" : {
                  "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248" : "0msat",
                  "023a41771d55140a27c35687727a4b51bc6b58254129bd216fb000f405b7701cb4" : "20000000msat"
               },
               "msatoshi_to_us" : 18798965,
               "to_us_msat" : "18798965msat",
               "msatoshi_to_us_min" : 18798965,
               "min_to_us_msat" : "18798965msat",
               "msatoshi_to_us_max" : 20000000,
               "max_to_us_msat" : "20000000msat",
               "msatoshi_total" : 20000000,
               "total_msat" : "20000000msat",
               "dust_limit_satoshis" : 546,
               "dust_limit_msat" : "546000msat",
               "max_htlc_value_in_flight_msat" : 18446744073709551615,
               "max_total_htlc_in_msat" : "18446744073709551615msat",
               "their_channel_reserve_satoshis" : 546,
               "their_reserve_msat" : "546000msat",
               "our_channel_reserve_satoshis" : 546,
               "our_reserve_msat" : "546000msat",
               "spendable_msatoshi" : 18252965,
               "spendable_msat" : "18252965msat",
               "htlc_minimum_msat" : 0,
               "minimum_htlc_in_msat" : "0msat",
               "their_to_self_delay" : 6,
               "our_to_self_delay" : 144,
               "max_accepted_htlcs" : 483,
               "status" : [
                  "CHANNELD_NORMAL:Reconnected, and reestablished.", 
                  "CHANNELD_NORMAL:Funding transaction locked.  Channel announced." 
               ],
               "in_payments_offered" : 0,
               "in_msatoshi_offered" : 0,
               "in_offered_msat" : "0msat",
               "in_payments_fulfilled" : 0,
               "in_msatoshi_fulfilled" : 0,
               "in_fulfilled_msat" : "0msat",
               "out_payments_offered" : 2,
               "out_msatoshi_offered" : 2402049,
               "out_offered_msat" : "2402049msat",
               "out_payments_fulfilled" : 1,
               "out_msatoshi_fulfilled" : 1201035,
               "out_fulfilled_msat" : "1201035msat",
               "htlcs" : []
            }
         ]
      }
   ]
}

Update 1
I tried sendpay.  Then it worked. 
I executed getroute command and I expected the fee was 1022. 
After I paid, the amount was decreased 1201022 (the invoice amount + fee) as I expected. 
18798965 - 17597943 = 1201022

// before payment
"msatoshi_to_us" : 18798965,

// after payment
"msatoshi_to_us" : 17597943,

// listfunds before payment
$ lightning-cli listfunds
{
   "outputs" : [
      {
         "txid" : "9c72f084d300898e566b96f049611fcd1a58d8fc498916e0e6bad2ff8ecb7366",
         "output" : 1,
         "value" : 92846,
         "amount_msat" : "92846000msat",
         "address" : "tb1qyee36f2jmsytxxymwzc8rps7e7gmkky04lvtz6",
         "status" : "confirmed"
      }
   ],
   "channels" : [
      {
         "peer_id" : "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248",
         "short_channel_id" : "1542376x20x0",
         "channel_sat" : 18798,
         "our_amount_msat" : "18798000msat",
         "channel_total_sat" : 20000,
         "amount_msat" : "20000000msat",
         "funding_txid" : "9c72f084d300898e566b96f049611fcd1a58d8fc498916e0e6bad2ff8ecb7366"
      }
   ]
}

// listpeers before payment
$ lightning-cli listpeers
{
   "peers" : [
      {
         "id" : "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248",
         "connected" : true,
         "netaddr" : [
            "23.237.77.12:9735"
         ],
         "globalfeatures" : "",
         "localfeatures" : "82",
         "channels" : [
            {
               "state" : "CHANNELD_NORMAL",
               "scratch_txid" : "e7a65a4f1e625a218baf640a154b04bdf06aa922ff1970ee64cbb3f0046b7866",
               "owner" : "lightning_channeld",
               "short_channel_id" : "1542376x20x0",
               "direction" : 1,
               "channel_id" : "6673cb8effd2bae6e0168949fcd8581acd1f6149f0966b568e8900d384f0729c",
               "funding_txid" : "9c72f084d300898e566b96f049611fcd1a58d8fc498916e0e6bad2ff8ecb7366",
               "private" : false,
               "funding_allocation_msat" : {
                  "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248" : 0,
                  "023a41771d55140a27c35687727a4b51bc6b58254129bd216fb000f405b7701cb4" : 20000000
               },
               "funding_msat" : {
                  "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248" : "0msat",
                  "023a41771d55140a27c35687727a4b51bc6b58254129bd216fb000f405b7701cb4" : "20000000msat"
               },
               "msatoshi_to_us" : 18798965,
               "to_us_msat" : "18798965msat",
               "msatoshi_to_us_min" : 18798965,
               "min_to_us_msat" : "18798965msat",
               "msatoshi_to_us_max" : 20000000,
               "max_to_us_msat" : "20000000msat",
               "msatoshi_total" : 20000000,
               "total_msat" : "20000000msat",
               "dust_limit_satoshis" : 546,
               "dust_limit_msat" : "546000msat",
               "max_htlc_value_in_flight_msat" : 18446744073709551615,
               "max_total_htlc_in_msat" : "18446744073709551615msat",
               "their_channel_reserve_satoshis" : 546,
               "their_reserve_msat" : "546000msat",
               "our_channel_reserve_satoshis" : 546,
               "our_reserve_msat" : "546000msat",
               "spendable_msatoshi" : 18252965,
               "spendable_msat" : "18252965msat",
               "htlc_minimum_msat" : 0,
               "minimum_htlc_in_msat" : "0msat",
               "their_to_self_delay" : 6,
               "our_to_self_delay" : 144,
               "max_accepted_htlcs" : 483,
               "status" : [
                  "CHANNELD_NORMAL:Reconnected, and reestablished.", 
                  "CHANNELD_NORMAL:Funding transaction locked.  Channel announced." 
               ],
               "in_payments_offered" : 0,
               "in_msatoshi_offered" : 0,
               "in_offered_msat" : "0msat",
               "in_payments_fulfilled" : 0,
               "in_msatoshi_fulfilled" : 0,
               "in_fulfilled_msat" : "0msat",
               "out_payments_offered" : 2,
               "out_msatoshi_offered" : 2402049,
               "out_offered_msat" : "2402049msat",
               "out_payments_fulfilled" : 1,
               "out_msatoshi_fulfilled" : 1201035,
               "out_fulfilled_msat" : "1201035msat",
               "htlcs" : []
            }
         ]
      },
      {
         "id" : "03f44acc1df8639efbe5099da403f213a4ce5bbe6f9bdac7bfa26dcc338761e9de",
         "connected" : true,
         "netaddr" : [
            "207.154.224.115:9735"
         ],
         "globalfeatures" : "",
         "localfeatures" : "81",
         "channels" : []
      }
   ]
}

// getroute
$ lightning-cli getroute 03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134 1200000 0
{
   "route" : [
      {
         "id" : "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248",
         "channel" : "1542376x20x0",
         "direction" : 1,
         "msatoshi" : 1201022,
         "amount_msat" : "1201022msat",
         "delay" : 157
      },
      {
         "id" : "034fe52e98a0e9d3c21b767e1b371881265d8c7578c21f5afd6d6438da10348b36",
         "channel" : "1514318x209x0",
         "direction" : 0,
         "msatoshi" : 1200021,
         "amount_msat" : "1200021msat",
         "delay" : 13
      },
      {
         "id" : "03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134",
         "channel" : "1513532x23x1",
         "direction" : 0,
         "msatoshi" : 1200000,
         "amount_msat" : "1200000msat",
         "delay" : 9
      }
   ]
}

// decodepay to get payment_hash
$ lightning-cli decodepay lntb12u1pwsp655pp5rs20fvw8es376wcxxv6u5lxgmh020tlaj77k57rwuzee8qd9a6jqdp2xys9xct5da3kx6twv9kk7m3qg3hkccm9ypxxzar5v5cqp5ufncstk7609dzjg5pm9vjsnf8aphvncpzndup9ly640ggzev76yyazzaxx5uachlp0ld62qlf4qvp7a9w7m4lyagwvehh5cjh2gr90sqet6hjs
{
   "currency" : "tb",
   "created_at" : 1560341140,
   "expiry" : 3600,
   "payee" : "03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134",
   "msatoshi" : 1200000,
   "amount_msat" : "1200000msat",
   "description" : "1 Satoccinamon Dolce Latte",
   "min_final_cltv_expiry" : 20,
   "payment_hash" : "1c14f4b1c7cc23ed3b063335ca7cc8dddea7affd97bd6a786ee0b39381a5eea4",
   "signature" : "3045022100e267882eded3cad149140ecac942693f43764f0114dbc097e4d55e840b2cf68802204e885d31a9cee2ff0bfedd281f4d40c0fba577b75f93a873337bd312ba9032be"
}

// sendpay
$ lightning-cli sendpay '[{"id" : "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248","channel" : "1542376x20x0","direction" : 1,"msatoshi" : 1201022,"amount_msat" : "1201022msat","delay" : 157},{"id" : "034fe52e98a0e9d3c21b767e1b371881265d8c7578c21f5afd6d6438da10348b36","channel" : "1514318x209x0","direction" : 0,"msatoshi" : 1200021,"amount_msat" : "1200021msat","delay" : 13},{"id" : "03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134","channel" : "1513532x23x1","direction" : 0,"msatoshi" : 1200000,"amount_msat" : "1200000msat","delay" : 9}]' 1c14f4b1c7cc23ed3b063335ca7cc8dddea7affd97bd6a786ee0b39381a5eea4
{
   "message" : "Monitor status with listpayments or waitsendpay",
   "id" : 2,
   "payment_hash" : "1c14f4b1c7cc23ed3b063335ca7cc8dddea7affd97bd6a786ee0b39381a5eea4",
   "destination" : "03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134",
   "msatoshi" : 1200000,
   "amount_msat" : "1200000msat",
   "msatoshi_sent" : 1201022,
   "amount_sent_msat" : "1201022msat",
   "created_at" : 1560342528,
   "status" : "pending"
}

// check the status of the payment
$ lightning-cli listpayments
{
   "payments" : [
      {
         "id" : 1,
         "payment_hash" : "a6e482c538b6e2469a0d47c35319783b3987b3c542aa8dc4623dfca91d288a96",
         "destination" : "03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134",
         "msatoshi" : 1200000,
         "amount_msat" : "1200000msat",
         "msatoshi_sent" : 1201035,
         "amount_sent_msat" : "1201035msat",
         "created_at" : 1560268918,
         "status" : "complete",
         "payment_preimage" : "c718e470dcf429280c611afba095c7396e219d60e590c6ee6beca8042f616483",
         "bolt11" : "lntb12u1pw0ljy4pp55mjg93fckm3ydxsdglp4xxtc8vuc0v79g24gm3rz8h72j8fg32tqdp2xys9xct5da3kx6twv9kk7m3qg3hkccm9ypxxzar5v5cqp5336eqcvsmkx9507p6r82ekralj82jl3npv0v2k8auj2kkgmc9l68mmmzkxvpcnzd3feyygp8hw6wq2qup6f97zfjlm2hzhlk4a9veegqstahde"
      },
      {
         "id" : 2,
         "payment_hash" : "1c14f4b1c7cc23ed3b063335ca7cc8dddea7affd97bd6a786ee0b39381a5eea4",
         "destination" : "03933884aaf1d6b108397e5efe5c86bcf2d8ca8d2f700eda99db9214fc2712b134",
         "msatoshi" : 1200000,
         "amount_msat" : "1200000msat",
         "msatoshi_sent" : 1201022,
         "amount_sent_msat" : "1201022msat",
         "created_at" : 1560342528,
         "status" : "complete",
         "payment_preimage" : "92ef9e7fd0d7d54565f83e623b8182fdb2884cc6c81071cb8ef3797da4a7efd5"
      }
   ]
}

// listfunds after payment
$ lightning-cli listfunds
{
   "outputs" : [
      {
         "txid" : "9c72f084d300898e566b96f049611fcd1a58d8fc498916e0e6bad2ff8ecb7366",
         "output" : 1,
         "value" : 92846,
         "amount_msat" : "92846000msat",
         "address" : "tb1qyee36f2jmsytxxymwzc8rps7e7gmkky04lvtz6",
         "status" : "confirmed"
      }
   ],
   "channels" : [
      {
         "peer_id" : "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248",
         "short_channel_id" : "1542376x20x0",
         "channel_sat" : 17597,
         "our_amount_msat" : "17597000msat",
         "channel_total_sat" : 20000,
         "amount_msat" : "20000000msat",
         "funding_txid" : "9c72f084d300898e566b96f049611fcd1a58d8fc498916e0e6bad2ff8ecb7366"
      }
   ]
}

// listpeers after payment
$ lightning-cli listpeers
{
   "peers" : [
      {
         "id" : "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248",
         "connected" : true,
         "netaddr" : [
            "23.237.77.12:9735"
         ],
         "globalfeatures" : "",
         "localfeatures" : "82",
         "channels" : [
            {
               "state" : "CHANNELD_NORMAL",
               "scratch_txid" : "743a7677371cea3cb4b75efdb4934ff3a1a8bdce9a152cf9266b057b24c6f300",
               "owner" : "lightning_channeld",
               "short_channel_id" : "1542376x20x0",
               "direction" : 1,
               "channel_id" : "6673cb8effd2bae6e0168949fcd8581acd1f6149f0966b568e8900d384f0729c",
               "funding_txid" : "9c72f084d300898e566b96f049611fcd1a58d8fc498916e0e6bad2ff8ecb7366",
               "private" : false,
               "funding_allocation_msat" : {
                  "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248" : 0,
                  "023a41771d55140a27c35687727a4b51bc6b58254129bd216fb000f405b7701cb4" : 20000000
               },
               "funding_msat" : {
                  "02312627fdf07fbdd7e5ddb136611bdde9b00d26821d14d94891395452f67af248" : "0msat",
                  "023a41771d55140a27c35687727a4b51bc6b58254129bd216fb000f405b7701cb4" : "20000000msat"
               },
               "msatoshi_to_us" : 17597943,
               "to_us_msat" : "17597943msat",
               "msatoshi_to_us_min" : 17597943,
               "min_to_us_msat" : "17597943msat",
               "msatoshi_to_us_max" : 20000000,
               "max_to_us_msat" : "20000000msat",
               "msatoshi_total" : 20000000,
               "total_msat" : "20000000msat",
               "dust_limit_satoshis" : 546,
               "dust_limit_msat" : "546000msat",
               "max_htlc_value_in_flight_msat" : 18446744073709551615,
               "max_total_htlc_in_msat" : "18446744073709551615msat",
               "their_channel_reserve_satoshis" : 546,
               "their_reserve_msat" : "546000msat",
               "our_channel_reserve_satoshis" : 546,
               "our_reserve_msat" : "546000msat",
               "spendable_msatoshi" : 17051943,
               "spendable_msat" : "17051943msat",
               "htlc_minimum_msat" : 0,
               "minimum_htlc_in_msat" : "0msat",
               "their_to_self_delay" : 6,
               "our_to_self_delay" : 144,
               "max_accepted_htlcs" : 483,
               "status" : [
                  "CHANNELD_NORMAL:Reconnected, and reestablished.", 
                  "CHANNELD_NORMAL:Funding transaction locked.  Channel announced." 
               ],
               "in_payments_offered" : 0,
               "in_msatoshi_offered" : 0,
               "in_offered_msat" : "0msat",
               "in_payments_fulfilled" : 0,
               "in_msatoshi_fulfilled" : 0,
               "in_fulfilled_msat" : "0msat",
               "out_payments_offered" : 3,
               "out_msatoshi_offered" : 3603071,
               "out_offered_msat" : "3603071msat",
               "out_payments_fulfilled" : 2,
               "out_msatoshi_fulfilled" : 2402057,
               "out_fulfilled_msat" : "2402057msat",
               "htlcs" : []
            }
         ]
      },
      {
         "id" : "03f44acc1df8639efbe5099da403f213a4ce5bbe6f9bdac7bfa26dcc338761e9de",
         "connected" : true,
         "netaddr" : [
            "207.154.224.115:9735"
         ],
         "globalfeatures" : "",
         "localfeatures" : "81",
         "channels" : []
      }
   ]
}

A:

As you mentioned your question is almost a duplicate of Why does my c-lightning node pay more than the invoice plus routing fees? 
But you asked if there is a way of knowing how much you will overpay before you pay.  
There are two ways :

You can configure lightningd with a maximum allowed fee for a payment.  In that case your node won't use a route with higher fees.  However you still don't know the exact routing fee.  
You can use sendpay command and specify the onion yourself.  You might take a prepared onion from getroute command.  In this way you have full control.  

From lightning-cli - - help
Sendpay route payment_hash [label] [msatoshi] [bolt11]
  Send along {route} in return for preimage of {payment_hash}

