We lease 465 retail sites from third parties under certain non-cancelable operating leases that expire from time to time through 2033. The weighted-average remaining lease term was 5.6 years as of December 31, 2020.

Lease expense as measured under ASC 842 was classified in the statement of income as follows (in thousands):

	For the Year Ended December 31,
	2020		2019
Cost of sales	$	25,214	$	27,493
Operating expenses		9,067		379
General and administrative expenses		1,081		685
Total	$	35,362	$	28,557

Variable lease payments included in the above table are based on inflation, revenues or volumes and totaled $2.3 million and $1.8 million for 2020 and 2019, respectively. Short-term lease payments included in the table above that are excluded from the lease liability amounted to $0.8 million and $0.6 million for 2020 and 2019, respectively. Cash paid for amounts included in the measurement of lease liabilities under operating leases totaled $33.1 million and $25.8 million for 2020 and 2019, respectively.

Lease expense as measured under ASC 840 was $21.5 million for 2018. Contingent rent expense, based on gallons sold, as measured under ASC 840 was $1.9 million for 2018.

As of December 31, 2020, future minimum rental payments under operating leases, excluding variable lease payments or short-term payments, were as follows (in thousands). The weighted-average discount rate as of December 31, 2020 was 6.7%.

2021	$	33,118
2022		31,350
2023		29,141
2024		26,224
2025		23,961
Thereafter		81,043
Total future payments		224,837
Less impact of discounting		51,499
		173,338
Current portion		31,958
Long-term portion	$	141,380

Most lease agreements include provisions for renewals. We generally do not include renewal options in our lease term for purposes of measuring our lease liabilities and right-of-use assets unless the sublease to our customer extends beyond the term of the head lease.

See Note 5 for information regarding the acquisition of leasehold interests in connection with the acquisition of retail and wholesale assets.

Of our leased sites, we operate 108 of them as company operated sites. Substantially all the remaining leased sites are subleased to lessee dealers or commission agents under leases with terms generally ranging from one to ten years and which may include renewal options. Sublease rental income amounted to $34.8 million, $38.2 million and $37.1 million for 2020, 2019 and 2018, respectively.

Motor fuel stations are leased to tenants under operating leases with various expiration dates ranging through 2032. Most lease agreements include provisions for renewals. We generally do not include renewal options in our lease term. Future minimum rental payments under non-cancelable operating leases with third parties as of December 31, 2020 were as follows (in thousands):

2021	$	48,667
2022		39,367
2023		27,646
2024		22,288
2025		17,505
Thereafter		38,902
Total future minimum lease payments	$	194,375

The future minimum rental payments presented above do not include contingent rent based on future inflation, future revenues or volumes of the lessee, or non-lease components for amounts that may be received as tenant reimbursements for certain operating costs.

Deferred rent income from straight-line rent relates to the cumulative amount by which straight-line rental income recorded to date exceeds cash rents billed to date under the lease agreement and totaled $5.3 million and $7.1 million at December 31, 2020 and 2019, respectively. See Note 5 for information regarding the write-off of deferred rent income as a result of terminating leases in connection with the acquisition of retail and wholesale assets.