Write a PROLOG Program to Find the Sum of first N natural numbers

predicates
add_upto(integer, integer)
clauses
add_upto(1, 1)
add_upto(N, Result) if
N>=2,
N_1=N-1,
add_upto(N_1, Res),
Result=Res+N.
goal
add_upto(3, Result)
Result=6