easy quick question - validation

Hello Experts,

Can somebody explain me on beginner level how should I rewrite this code? I'm trying to do validation for char variable with while loop.

do {
cout << "Please enter your sex\n";
cout << "\t M - Male\n";
cout << "\t F - Female\n";
cout << " ==> ";
cin >> s;
} while ( s != 'f' );

this works but just for 'f' but when I add "or"

do {
cout << "Please enter your sex\n";
cout << "\t M - Male\n";
cout << "\t F - Female\n";
cout << " ==> ";
cin >> s;
} while ( s != 'f' || s != 'F' || s != 'm' || s != 'M');

thanks