DETROIT — Matthew Stafford’s first game under Jim Caldwell was a success.Of course, it helps to have a healthy Calvin Johnson running free behind the opposing secondary.Stafford improvised brilliantly on two early touchdown passes to Johnson, and Detroit beat the New York Giants 35-14 on Monday night in Caldwell’s first game as Lions coach.Stafford also ran for a TD for Detroit, which intercepted Eli Manning twice in the Lions’ first Monday night opener since 1971. It was a bright start for Detroit, which collapsed down the stretch last year and missed the postseason. Jim Schwartz was fired and replaced by Caldwell.Stafford went 22 of 32 for 346 yards.