n=1000; sum=0; y=rep(0,n)
for(i in 1:n){
 y1=rbinom(1,1,0.3)
 y2=rbinom(1,1,0.3)
 if(y1==y2){sum=sum+1}
 y[i]=sum
 cat("fly 1 is",ifelse(y1==1,"grey ","black"),
   "; fly 2 is",ifelse(y2==1,"grey ","black"),
   "; same?",ifelse(y1==y2,"yes","no "),
"; prop=",sum,"/",i,"=",sum/i,"\n")
}

plot(y/1:n,ylim=c(0,1))
abline(a=0.58,b=0)