#APL(NARS), 15 chars, 30 bytes
{+/∊(⍵⍴2)⊤⍺..⍵}
test
f←{+/∊(⍵⍴2)⊤⍺..⍵}
1000 f 2000
5938
1 of 3
user58988
#APL(NARS), 15 chars, 30 bytes
{+/∊(⍵⍴2)⊤⍺..⍵}
test
f←{+/∊(⍵⍴2)⊤⍺..⍵}
1000 f 2000
5938