Let a = answer
Let n_0 = n
Let n_a = n+(a-1)

Problem => (n_0 + n_a) * a / 2 > m, find a
Feed the formula to wolframalpha(google it youself)

Convert it to code, ac.

Hint 1 math.h

Hint 2 check if n>m

solve (n+n+a-1) * a / 2 > m , a