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OMAHA, Neb. (AP) — USG Corp. says a $5.9 billion takeover offer from rival building products manufacturer Knauf is inadequate.

USG said Monday the German company’s offer significantly undervalues the Chicago construction materials company. USG shares jumped 19 percent to $39.92.

USG Chairman Steven Leer says the company is confident that the strategy it outlined for investors earlier this month will deliver more to shareholders than Knauf’s offer would.

Knauf says it’s waiting for a formal response to its offer.

Investor Warren Buffett’s Berkshire Hathaway Omaha, Nebraska, conglomerate is USG’s biggest shareholder with 31 percent.

Berkshire says it offered to sell Knauf options to buy its 43 million shares for at least $42 a share if it reaches a deal to buy USG in the next six months.