Sum of interior angles = (n - 2) * 180 = 180n - 360.....where n = the number of sides
So
180n - 360 = 1780
180n = 1780 + 360
180n = 2140
We need n to be a whole number > 2140 / 180
So
ceiling (2140 / 180) = n = 12
So
(12 - 2) 180 =
10 * 180 =
1800
The missing angle = 1800 - 1780 = 20°