Drummer Larry Mullen Jr., a founding member of Irish rock band U2, has revealed that he has a learning difficulty that makes counting musical bars feel like climbing Mount Everest.

“I’ve always known that there’s something not particularly right with the way that I deal with numbers. I’m numerically challenged,” the Grammy Award-winning musician told Britain’s Times Radio in an interview to be released later Friday. Its sister outlet, the Times newspaper, published excerpts from the interview on Thursday.

“And I realised recently that I have dyscalculia, which is a sub-version of dyslexia. So I can’t count (and) I can’t add,” he said.

According to the British Dyslexia Association website, around 6% of the United Kingdom’s population have dyscalculia, which is a specific and persistent difficulty in understanding numbers,. A similar percentage of students have the condition in the US, according to the Learning Disabilities Association of America website.

“When people watch me play sometimes, they say, ‘You look pained.’ I am pained because I’m trying to count the bars,” Mullen said.

“I had to find ways of doing this – and counting bars is like climbing Everest,” he continued.

Mullen, age 63, opened up about his recent diagnosis while talking about his upcoming documentary film “Left Behind,” which explores the story of a group of frustrated and determined moms who work together to open the first public school for dyslexic students in New York City.

His eldest son has dyslexia, a learning difficulty that affects people’s ability to read, write and spell, according to the film’s website.

Mullen achieved global fame in the 1980s as part of U2, which he formed with his high school friends, singer Bono, bassist Adam Clayton and guitarist and keyboard player the Edge, in Dublin in 1977.

The band, which entered the Rock and Roll Hall of Fame in 2005, is known for hits such as “With or Without You,” “I Still Haven’t Found What I’m Looking For” and “Beautiful Day.”