In his provocative shareholder letter, JPMorgan Chase chief executive Jamie Dimon writes that long-term thinking is vital.

“Making important investments, in good times and not so good times,” has been key to Chase’s success, he writes. “These investments drive the future prospects of our company and position it to grow and prosper for decades.”

That letter prompted me to look at one of my musty old files, containing the history of how a tiny investment I made in a Detroit bank stock for journalistic reasons more than 40 years ago has morphed into JPMorgan shares.

I suddenly realized that acting Dimonesque by reinvesting dividends rather than taking them in cash has helped produce a return of more than 100 to 1 on my ancient investment. That’s right. More than 100 to 1.

The saga started Jan. 4, 1974, when I plunked down $40.50 to buy one share of National Detroit, which 30 years and three mergers later became part of Chase.

National Detroit was one of the firms I covered during my days as a Detroit Free Press banking reporter. I bought my share — with my editor’s permission — so I could get shareholder documents (which were sometimes difficult to come by back then) and to make sure I could get into the bank’s annual shareholder meeting. When I left the Free Press in 1979, a combination of inertia and nostalgia led me to keep my National Detroit stock and my shares in three other Detroit banks.

Neglect and sloth occasionally pay off. Over 43 years, several stock splits and three takeovers, my single National Detroit share has morphed into almost 45 shares of Chase that had a market value of about $3,860 when I did the math for this column.

When you add the 25 years of cash dividends that I got from National Detroit and its successor, First National NBD of Chicago, it’s clear that the cash I’ve gotten plus the value of my Chase shares total over 100 times my investment.

Part of the reason I’ve made such a large return is that the stock market and Detroit’s economy were depressed when I made my initial purchase in 1974.

But a major reason for my high return is that for 18 years, I’ve gone the Dimon route and reinvested.

That started in 1999, when Bank One bought First National NBD and offered me the option of reinvesting dividends rather than continuing to get four smallish checks a year that I had to remember to take to the bank.

Reinvesting those dividends increased my Bank One stake from an initial 31 shares in 1999 to 34-plus shares by the time Chase bought Bank One in 2004.

Reinvesting Chase dividends has increased my stake from an initial 32 shares to my current 44-plus. By my estimate, I’ve got 65 percent more Chase stock than I’d have if I’d taken my Bank One and Chase dividends in cash.

It helps — a lot — that the stock market was very strong between 1974 and the time I became a Bank One shareholder 25 years later. The Standard & Poor’s 500-stock index rose by 1,208 percent in that period, which is a really nice gain.

My National Detroit-First Chicago investment, though, had an even nicer gain. It rose about 2,700 percent, as best I can tell.

I’ve also been fortunate since embarking on the dividend-reinvestment route 18 years ago. During that period, the value of my Bank One-then-Chase shares has risen 236 percent, compared with 156 percent for the S&P 500 and 184 percent for investor-class shares of Vanguard’s Total Stock Market fund. The S&P and Vanguard numbers include price increases and reinvested dividends, just as my Chase calculations do.

The bottom line: Most CEO shareholder letters are so boring I can use them as a sleep aid. But Dimon’s made me think and even generated a column idea. Yet another bonus return on my investment all those years ago.