python温度转换提示“TypeError: string indices must be integers”

TempStr=(input("请输入带有符号的温度值:"))
if TempStr[-1] in ['F','f']:
    C = (eval(TempStr[0,-1] - 32))/1.8
    print("转换后的温度是{:.2f}C".format(C))
elif TempStr[-1] in['C','c']:
    F = 1.8*eval(TempStr[0,-1]) + 32
    print("转换后的温度是{:.2f}F".format(F))
else:
    print("输入错误")

TypeError: string indices must be integers

环境python3.7.2。麻烦大神帮忙看看，新手。

C = (eval(TempStr[:-1] - 32))/1.8
F = 1.8*eval(TempStr[:-1]) + 32