ski slope: $0...$N

snow making machine at the ski slope. each machine is tuple of $($index$,$ range$)$ indicating machine is on the slope at index and is capable of covering the slope with snow up to range indices away. Find smalled number of snow machines that need to be turned on to cover entire slope. if not possible return $-1$

Example:

N$=2,$ machines : $[(0,1),(1,1),(2,1)]$ # output $1->$ need to turn on middle machine

N$=3,$ machines $=[(0,2)]$ # output $-1$

N$=5,$ machines$=[(0,2),(1,1),(3,1),(4,1)]$ #output: $2->$ either machine at $0$r index $1$ and machine at $4$th index