using string in one way selection (if statement)

please help me to know how to use the string s1, to print success when StdMark>50 in this code. Thank you

#include <iostream>
#include <string>
using namespace std;

int main() 
{
    int StdMark;
    string s1;
    s1="succuss";
    cout <<"Enter The grade:"<<endl;
    cin >>StdMark;
    s
    if(StdMark<50)
    {
    cout<<"fail";
}
    return 0;
}