Getting mathematical function as user's input in Python

Question

I used f = lambda x: input() to get a mathematical function as user's input, but when I put this in a for loop, the function is requested in every iterating. How do I store the function in a variable only once and use it in the loop?
from math import cos, sin, pow, e
n = int(input('Number of elements in partition: '))
print('The range is set at ...... ')
minimum = int(input('Minimum: '))
maximum = int(input('Maximum: '))
f = lambda x: eval(input('Enter a funtion: '))
fraction = (maximum - minimum) / (n-1)
UpperSum = 0
LowerSum = 0
for i in range(1, n):
UpperSum = UpperSum + fraction * f(fraction * i)
LowerSum = LowerSum + fraction * f(fraction * (i - 1))
print(f'UpperSum: {UpperSum}\nLowerSum {LowerSum}')