What was Olivia Newton-John's first #1 song in the U.S.?

Getting a #1 hit in the '70s meant going head-to-head with some serious competition. The charts were packed with everything from Barry White's disco soul to Barbra Streisand's theatrical ballads. But for someone like Olivia Newton-John, it was always a question of when, not if, she'd top the Billboard Hot 100. What was her first #1 song in the U.S.?