foo.cc: In function 'int main()':
foo.cc:19:15: warning: pointer to a function used in arithmetic [-Wpointer-arith]
    if(s(i)+s[j]+4+s[i+j]==n)
               ^
foo.cc:19:11: warning: pointer to a function used in arithmetic [-Wpointer-arith]
    if(s(i)+s[j]+4+s[i+j]==n)
       ~~~~^~~~~
foo.cc:19:16: warning: pointer to a function used in arithmetic [-Wpointer-arith]
    if(s(i)+s[j]+4+s[i+j]==n)
       ~~~~~~~~~^~
foo.cc:19:24: warning: pointer to a function used in arithmetic [-Wpointer-arith]
    if(s(i)+s[j]+4+s[i+j]==n)
                        ^
foo.cc:19:18: error: invalid operands of types 'int (*)(int)' and 'int(int)' to binary 'operator+'
    if(s(i)+s[j]+4+s[i+j]==n)
       ~~~~~~~~~~~^~~~~~~